Ultra‐thin,high performance tunnel junctions for III–V multijunction cells

Tunnel junctions (TJ) made of p‐Al_0.1 Ga_0.9As/n‐GaAs are used because of their high peak current and low series resistance, but are not fully transparent. The influence of reducing the thickness of these tunnel junctions on the characteristics of InGaP/GaAs tandem cells was investigated. It was found that ultra‐thin TJs with excellent performance can be realized. Even for a 7.5/6‐nm thick TJ, which is the thinnest possible in our growth reactor, the peak current density is at least 600 A/cm². The series resistance of the TJs was found to be at a constant level of 0.6 ± 0.2 mΩ cm² for all total thicknesses of the TJ in the 13.5–40 nm range. Because of a lower absorption in the TJ, a tandem cell with a 7.5/6‐nm thick TJ, compared with a cell with a 20/20‐nm thick TJ, gained 0.53 ± 0.05 mA/cm² in short circuit current to a value of 14.8 mA/cm². Copyright © 2012 John Wiley & Sons, Ltd.     

Fabrication and analysis of multijunction solar cells with a quantum dot (In)GaAs junction

InAs quantum dots (QDs) have been incorporated to bandgap engineer the (In)GaAs junction of (In)GaAs/Ge double‐junction solar cells and InGaP/(In)GaAs/Ge triple‐junction solar cells on 4‐in. wafers. One sun AM0 current–voltage measurement shows consistent performance across the wafer. Quantum efficiency analysis shows similar aforementioned bandgap performance of baseline and QD solar cells, whereas integrated sub‐band gap current of 10 InAs QD layers shows a gain of 0.20 mA/cm². Comparing QD double‐junction solar cells and QD triple‐junction solar cells to baseline structures shows that the (In)GaAs junction has a V_oc loss of 50 mV and the InGaP 70 mV. Transmission electron microscopy imaging does not reveal defective material and shows a buried QD density of 10¹¹ cm⁻², which is consistent with the density of QDs measured on the surface of a test structure. Although slightly lower in efficiency, the QD solar cells have uniform performance across 4‐in. wafers. Copyright © 2013 John Wiley & Sons, Ltd.     

GaAs solar cells grown on Si substrates for space use

GaAs single‐junction and InGaP/GaAs multi‐junction thin‐film solar cells fabricated on Si substrates have great potential for high‐efficiency, low‐cost, lightweight and large‐area space solar cells. Heteroepitaxy of GaAs thin films on Si substrates has been examined and high‐efficiency GaAs thin‐film solar cells with total‐area efficiencies of 18·3% at AM0 and 20·0% at AM 1·5 on Si substrates (GaAs‐on‐Si solar cells) have been fabricated. In addition, 1‐MeV electron irradiation damage to GaAs‐on‐Si cells has been studied. The GaAs‐on‐Si cells are found to show higher end‐of‐life efficiency than the conventional GaAs cells fabricated on GaAs substrates (GaAs‐ on‐GaAs cells) under high‐fluence 1‐MeV electron irradiation of more than 1 × 10¹⁵ cm⁻². The first space flight to make use of them has been carried out. Forty‐eight 2 × 2 cm GaAs‐on‐Si cells with an average AM0 total‐area efficiency of 16·9% have been evaluated in the Engineering Test Satellite No.6 (ETS‐VI). The GaAs‐on‐Si cells have been demonstrated to be more radiation‐resistant in space than GaAs‐on‐GaAs cells and 50, 100 and 200‐μm‐thick Si cells. These results show that the GaAs‐on‐Si single‐junction and InGaP/GaAs‐on‐Si multi‐junction cells have great potential for space applications. Copyright © 2001 John Wiley & Sons, Ltd.     

Comparison of OMVPE Grown GaAs/AlGaAs and GaAs/InGaP HEMT and PHEMT structures

Symmetric δ-doped InGaP and AlGaAs PHEMT structures have been grown by organometallic vapor phase epitaxy with properties that approach those of MBE grown AlGaAs structures. The 300 and 77K carrier concentrations for the InGaP PHEMT were 2.72 and 2.56 × 10¹² cm^{2 −2} and the mobilities were 5,920 and 22,000 cm^{2 2}/V.s. These excellent values suggest that problems associated with switching the anion at the channel heterojunction have been overcome. The corresponding values for the AlGaAs PHEMT were 2.51 and 2.19 × 10¹² cm^{2 −2} and 6,500 and 20,400 cm²/V.s. The uniformity in the indium concentration in the InGaAs layer as determined by photoluminescence, photoreflection, double crystal x-ray diffraction, and Rutherford backscattering was found to be good, but the percent In in the AlGaAs pseudo-morphic high electron mobility transistor (PHEMT) was less than that in the InGaP PHEMT even though the programmed values were the same. The uniformity in the doping distribution as determined by secondary ion mass spectroscopy and electrochemical capacitance-voltage measurements was found to be good, but it decreased with distance from the center of the susceptor. Also, most of the dopants in the δ-doped InGaP and AlGaAs layers were activated.     

Influence of double- and triple-layer antireflection coatings on the formation of photocurrents in multijunction III–V solar cells

The results of simulation by the transfer-matrix method of TiO₂/SiO₂ double-layer and TiO₂/Si₃N₄/SiO₂ triple-layer antireflection coatings for multijunction InGaP/GaAs/Ge heterostructure solar cells are presented. The TiO₂/SiO₂ double-layer antireflection coating is experimentally developed and optimized. The experimental spectral dependences of the external quantum yield of the InGaP/GaAs/Ge heterostructure solar cell and optical characteristics of antireflection coatings, obtained in the simulation, are used to determine the photogenerated current densities of each subcell in the InGaP/GaAs/Ge solar cell under AM1.5D irradiation conditions (1000 W/m²) and for the case of zero reflection loss. It is shown in the simulation that the optimized TiO₂/Si₃N₄/SiO₂ triple-layer antireflection coating provides a 2.3 mA/cm² gain in the photocurrent density for the Ge subcell under AM1.5D conditions in comparison with the TiO₂/SiO₂ double-layer antireflection coating under consideration. This thereby provides an increase in the fill factor of the current–voltage curve and in the output electric power of the multijunction solar cell.     

Current gain dependence on subcollector and etch-stop doping inInGaP/GaAs HBTs

The DC current gain dependence of InGaP/GaAs heterojunction bipolar transistors (HBTs) on subcollector and etch-stop doping is examined. Samples of InGaP/GaAs HBTs having various combinations of subcollector doping and etch-stop doping are grown, and large area 60 μm×60 (μ) HBTs are then fabricated for DC characterization. It is found that the DC current gain has a strong dependence on the doping concentration in the subcollector and the subcollector etch-stop. Maximum gain is achieved when the subcollector is doped at 6~7×10 ¹⁸ cm^-3 while the subcollector etch-stop is doped either above 6×10¹⁸ cm^-3 (current gain/sheet resistance ratio, β/R_b=0.435 at I_c=1 mA) or below 3.5×10¹⁷ cm^-3 (β/R_b=0.426~0.438 at I_c=1 mA). The data show that it is not necessary to heavily dope the subcollector etch-stop to reduce the conduction barrier and to obtain high current gain. The high current gain obtained with the low InGaP etch-stop doping concentration is attributed to the reduction of the effective energy barrier thickness due to band bending at the heterojunction between the InGaP etch-stop and the GaAs subcollector. These results show that the β/R_b of InGaP/GaAs HBTs can improve as much as 69% with the optimized doping concentration in subcollector and subcollector etch-stop     

Enhanced lateral current transport via the front N+ diffused layer of n‐type high‐efficiency back‐junction back‐contact silicon solar cells

N‐type back‐contact back‐junction solar cells were processed with the use of industrially relevant structuring technologies such as screen‐printing and laser processing. Application of the low‐cost structuring technologies in the processing of the high‐efficiency back‐contact back‐junction silicon solar cells results in a drastic increase of the pitch on the rear cell side. The pitch in the range of millimetres leads to a significant increase of the lateral base resistance. The application of a phosphorus doped front surface field (FSF) significantly reduces the lateral base resistance losses. This additional function of the phosphorus doped FSF in reducing the lateral resistance losses was investigated experimentally and by two‐dimensional device simulations. Enhanced lateral majority carrier's current transport in the front n⁺ diffused layer is a function of the pitch and the base resistivity. Experimental data show that the application of a FSF reduces the total series resistance of the measured cells with 3.5 mm pitch by 0.1 Ω cm² for the 1 Ω cm base resistivity and 1.3 Ω cm² for the 8 Ω cm base resistivity. Two‐dimensional simulations of the electron current transport show that the electron current density in the front n⁺ diffused layer is around two orders of magnitude higher than in the base of the solar cell. The best efficiency of 21.3% was obtained for the solar cell with a 1 Ω cm specific base resistivity and a front surface field with sheet resistance of 148 Ω/sq. Copyright © 2008 John Wiley & Sons, Ltd.     

基区设计对InGaP/GaAs HBT热稳定性的影响

研究了不同基区设计对多发射极指结构功率InGaP/GaAs异质结双极型晶体管热稳定性的影响。以发生电流增益崩塌的临界功率密度为热稳定性判定标准,推导了热电反馈系数Φ、集电极电流理想因子η和热阻Rth与基区掺杂浓度NB、基区厚度dB的理论公式。基于TCAD虚拟实验,观测了不同基区掺杂浓度和不同基区厚度分别对InGaP/GaAs HBT热稳定性的影响。结合理论公式,对仿真实验曲线进行了分析。结果表明,基区设计参数对热稳定性有明显的影响,其影响规律不是单调变化的。通过基区外延层参数的优化设计,可以改进多指HBT器件的热稳定性,从而为多指InGaP/GaAs HBT热稳定性设计提供了一个新的途径。     

Activity and current status of R&D on space solar cells in Japan

Japan's Research and Development (R&D) activities on high‐performance III–V compound space solar cells are presented. Studies of new CuInGaSe₂ thin‐film terrestrial solar cells for space applications are also discussed. Performance and radiation characteristics of a newly developed InGaP/GaAs/Ge triple‐junction space solar cell, including radiation response, results of a flight demonstration test of InGaP/GaAs dual‐junction solar cells and CuInGaSe₂ thin‐film solar cells, and radiation response of three component sub‐cells are explained. This study confirms superior radiation tolerance of InGaP/GaAs dual‐junction cells and CuInGaSe₂ thin‐film cells by space flight experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of InGaP heteropassivation on reliability of GaAs HBTs

For the self-aligned AlGaAs/GaAs HBTs with the mesa-etched emitter, the instability of the surface states on the extrinsic base passivated by nitride is a major cause of the severe degradation of current gain. In this paper GaAs HBTs employing InGaP ledge emitter in order to passivate the surface of the extrinsic base and to reduce the surface states exhibited the considerable improvement of the current gain reliability with the activation energy of 1.97 eV and MTTF of 4.8×10⁸ h at 140°C. However, under the strong stress conditions InGaP/GaAs HBTs also produced the considerable degradation. The possible origins were investigated.     

Enhanced efficiency in GaInP/GaAs tandem solar cells using carbon doped GaAs in tunnel junction

Carbon doping of GaAs using CBr₄ (carbon tetrabromide) in metal-organic chemical vapor deposition (MOCVD) was investigated to obtain very high and sharp doping profiles required for tunnel junction in tandem solar cells. It was found that the hole concentration increased with decreasing growth temperature and V/III ratio. Hole doping profiles versus distance from the sample surface showed that the hole concentration near the surface was very low in comparison with that far below the surface. As a post-growth treatment, CBr₄ was supplied during the cool down process and produced almost constant hole concentration of 1 × 10²⁰ cm⁻³ regardless of the depth, when CBr₄ flow rate was 9.53 μmol/min. Based on these results, solar cells were fabricated using both carbon (C) and zinc (Zn) as a p-type dopant. It was shown that C doping exhibits higher efficiency and lower series resistance than those of Zn doping in GaInP/GaAs tandem solar cells.     

Demonstration of a nipi‐diode photovoltaic

The simulation, growth, processing, and characterization of a three‐period GaAs n‐type/intrinsic/p‐type/intrinsic … (nipi) doping solar cell is demonstrated. A V‐groove etching process is characterized and used to expose the multiple n‐type and p‐type layers for electrical connection made by interdigitated grid‐finger electrodes. A five‐layer photolithographic process flow is developed and used to make 1 × 1 cm² devices with varying grid‐finger separation. Device simulations of the structure indicate that strong rectification can be achieved in the parallel‐connected three period nipi GaAs solar cell structure provided the necessary semiconductor doping compensation is achieved in the region near the metal‐semiconductor interfaces. Experimentally, the improvements observed in the open circuit voltage, short circuit current, and ideality of the devices following thermal annealing suggests the formation of barriers near the contacts, which support the simulation results. A comparison of the short circuit current and series resistance under illumination indicate a tradeoff between shadowing and series resistance, which may be overcome with modification to the device structure. Ultimately, these results show promise towards the development of high efficiency solar cells or radioisotope batteries, and offer a novel device structure for the incorporation of nano‐structures such as quantum wells or quantum dots. Copyright © 2010 John Wiley & Sons, Ltd.     

An Improved Theoretical Model to Explain Electronic and Optical Properties of p-Type GaAs/AlGaAs Superlattices for Multi-Wavelength Normal Incidence Photodetectors

We extend our previous theoretical analysis of electronic and optical properties of p-type quantum well structures based on the two heavy- and light-hole system to include all the three valence bands. These theories are then used to clarify the origin of the normal incidence absorption and photocurrent at photon wavelengths of 2 - 3 μm, which was observed in addition to the absorption around 8 μm by a recent experimental investigation with heavily doped p-type GaAs/AlGaAs multi-quantum well (MQW) structures. In the theoretical analysis, the Hartree and exchange-correlation many-body interactions are taken into account within one-particle local density approximation, and it is shown that normal incidence absorption occurs in two wavelength regions over the transition energy range higher than barrier height for p-type GaAs/AlGaAs superlattices with well doping of 2 × 10¹⁹ cm⁻³; one region has broad absorption peaks with coefficients of about 5000 cm⁻⁻¹ around 8 μm, and the other has two rather sharp peaks at 2.7 μm and 3.4 μm with 1800 cm⁻⁻¹ and 1300 cm⁻⁻¹, respectively. The result indicates that the theory explains the experimental observation well, as the theoretical and experimental results are in close agreement in general absorption features.     

Te掺杂超宽带隙隧穿结研究

研究制备得到峰值电流密度为65.3A/cm2的GaInP/AlGaAs宽带隙隧穿结和峰值电流密度为6.1A/cm2的AlGaInP/AlGaAs超宽带隙隧穿结。在隧穿结中使用二乙基碲（DETe）作为n型掺杂剂,实验中研究了材料生长温度、阀门开关处理以及DETe的流量等生长参数。采用预掺杂和升温后处理的方式来解决碲源的开关效应,而Te掺杂引入的晶格失配采用应力平衡技术来消除。另外研究了不同DETe流量下制备得到的隧穿结性能。     

Proton irradiation of CdTe thin film photovoltaics deposited on cerium‐doped space glass

Space photovoltaics is dominated by multi‐junction (III‐V) technology. However, emerging applications will require solar arrays with high specific power (kW/kg), flexibility in stowage and deployment, and a significantly lower cost than the current III‐V technology offers. This research demonstrates direct deposition of thin film CdTe onto the radiation‐hard cover glass that is normally laminated to any solar cell deployed in space. Four CdTe samples, with 9 defined contact device areas of 0.25 cm², were irradiated with protons of 0.5‐MeV energy and varying fluences. At the lowest fluence, 1 × 10¹² cm⁻², the relative efficiency of the solar cells was 95%. Increasing the proton fluence to 1 × 10¹³ cm⁻² and then 1 × 10¹⁴ cm⁻² decreased the solar cell efficiency to 82% and 4%, respectively. At the fluence of 1 × 10¹³ cm⁻², carrier concentration was reduced by an order of magnitude. Solar Cell Capacitance Simulator (SCAPS) modelling obtained a good fit from a reduction in shallow acceptor concentration with no change in the deep trap defect concentration. The more highly irradiated devices resulted in a buried junction characteristic of the external quantum efficiency, indicating further deterioration of the acceptor doping. This is explained by compensation from interstitial H⁺ formed by the proton absorption. An anneal of the 1 × 10¹⁴ cm⁻² fluence devices gave an efficiency increase from 4% to 73% of the pre‐irradiated levels, indicating that the compensation was reversible. CdTe with its rapid recovery through annealing demonstrates a radiation hardness to protons that is far superior to conventional multi‐junction III‐V solar cells.     

MOCVD-grown broad area InGaAs/GaAs/InGaP laser diodes

A MOCVD technology for growth of InGaAs/GaAs/InGaP laser heterostructures on a modified Epiquip VP-50-RP installation was developed. Mesa stripe laser diodes with threshold current density J _th=100–200 A/cm², internal optical loss α_i=1.3–1.7 cm^?1, and internal quantum efficiency η_i=60–70% have been fabricated. A CW output optical power of 5 W has been obtained for a single 100-µm-wide aperture mesa stripe laser diode emitting at 1.03 µm. It is shown that use of AlGaAs waveguide layers, which increase the conduction band barrier offset, lowers the temperature sensitivity of laser heterostructures within the temperature range 10–80°C.     

Transport parameters and optical properties of selectively doped Ga(Al)As/Zn(Mn)Se heterovalent structures with a two-dimensional hole channel

The growth of III–V/II–VI:Mn heterostructures with a high hole concentration in the AlGaAs:Be/GaAs/AlGaAs 2D channel situated in the immediate vicinity of the AlGaAs/Zn(Mn)Se heterovalent interface by molecular-beam epitaxy is reported. Despite the decrease in the hole concentration in the GaAs channel upon a decrease in the distance between the channel and the heterovalent interface, the hole concentration reaches a value of 1.5 × 10¹³ cm^?2 at a temperature of 300 K even at the minimum distance of 1.2 nm. Deep profiling by dynamic secondary-ion mass spectrometry confirmed the back diffusion of Mn from ZnMnSe into the III–V part. High hole concentration and the presence of magnetic manganese ions in the GaAs conduction channel determine the interest in the structures as possible objects in which the effect of magnetic ordering in heterogeneous semiconductor systems can be studied.     

Optical characteristics of InGaP/GaAs HPTs

In this paper, we examined the optical characteristics of InGaP/GaAs heterojunction phototransistor (HPT) directly compared with AlGaAs/GaAs HPT for the first time. Because of its inherent good electrical properties, the InGaP/GaAs HPT produced a high optical gain of about 61 at V_C=3 V, I_B=2 μA, for an input optical power of 1.23 μW. This is 2.5 times as high as that of the AlGaAs/GaAs HPT. In the transient response, the InGaP/GaAs HPT was a little inferior to the AlGaAs/GaAs HPT. This is due to the longer time delay caused by the photo-generated hole accumulation at the interface of heterojunction. The extended response time can be overcome by using a small load resistance in conjunction with the advantage of the superior optical gain     

Degradation and recovery of AlGaAs/GaAs p-HEMT irradiated by high-energy particle

Results of an extensive study on the irradiation damage and its recovery behavior resulting from thermal annealing in AlGaAs/GaAs pseudomorphic high electron mobility transistors (HEMTs) subjected to a 220-MeV carbon, 1-MeV electrons and 1-MeV fast neutrons are presented. The drain current and effective mobility decrease after irradiation, while the threshold voltage increases in positive direction. The decrease of the drain current and mobility is thought to be due to the scattering of channel electrons with the induced lattice defects and also to the decrease of the electron density in the two dimensional electron gas region. Isochronal thermal annealing shows that the device performance degraded by the irradiation recovers. The decreased drain current for output characteristics recovers by 75% of pre-rad value after 300°C thermal annealing for AlGaAs HEMTs irradiated by carbon particles with a fluence of 1×10¹² cm⁻². The influence of the materials and radiation source on the degradation is also discussed with respect to the nonionizing energy loss. Those are mainly attributed to the difference of particle mass and the probability of nuclear collision for the formation of lattice defect in Si-doped AlGaAs donor layer. A comparison is also made with results obtained on irradiated InGaP/InGaAs p-HEMTs in order to investigate the effect of the constituent atom. The damage coefficient of AlGaAs HEMTs is also about one order greater than that of InGaP HEMTs for the same radiation source. The materials and radiation source dependence of performance degradation is mainly thought to be attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects in Si-doped donor layer.     

Highly conductive p + + ‐AlGaAs/n + + ‐GaInP tunnel junctions for ultra‐high concentrator solar cells

Tunnel junctions are key for developing multijunction solar cells (MJSC) for ultra‐high concentration applications. We have developed a highly conductive, high bandgap p ^{+ +} ‐AlGaAs/n ^{+ +} ‐GaInP tunnel junction with a peak tunneling current density for as‐grown and thermal annealed devices of 996 A/cm ² and 235 A/cm ², respectively. The J–V characteristics of the tunnel junction after thermal annealing, together with its behavior at MJSCs typical operation temperatures, indicate that this tunnel junction is a suitable candidate for ultra‐high concentrator MJSC designs. The benefits of the optical transparency are also assessed for a lattice‐matched GaInP/GaInAs/Ge triple junction solar cell, yielding a current density increase in the middle cell of 0.506 mA/cm ² with respect to previous designs. Copyright © 2014 John Wiley & Sons, Ltd.