Growth and properties of digitally-alloyed AlGaInP by solid source molecular beam epitaxy

Digital alloying using molecular beam epitaxy (MBE) was investigated to produce AlGaInP quaternary alloys for bandgap engineering useful in 600-nm band optoelectronic device applications. Alternating Ga_0.51In_0.49P/Al_0.51In_0.49P periodic layers ranging from 4.4 monolayers (ML) to 22.4 ML were used to generate 4,000-Å-thick (Al_0.5Ga_0.5)_0.51In_0.49P quaternary materials to understand material properties as a function of constituent superlattice layer thickness. High-resolution x-ray diffraction (XRD) analysis exhibited fine satellite peaks for all the samples confirming that digitally-alloyed (Al_0.5Ga_0.5)_0.51In_0.49P preserved high structural quality consistent with cross-sectional transmission electron microscopy (X-TEM) images. Low-temperature photoluminescence (PL) measurements showing a wide span of luminescence energies ～ 170 meV can be obtained from a set of identical composition digitally-alloyed (Al_0.5Ga_0.5)_0.51In_0.49P with different superlattice periods, indicating the bandgap tunability of this approach and its viability for III-P optoelectronic devices grown by MBE.     

Hydrogen-related effects in GaInP/GaAs HBTs: incorporation, removal and influence on device reliability

We report the result of investigation on hydrogen effects on GaInP/GaAs HBT structures originating from different MOCVD and CBE suppliers. It is demonstrated that hydrogen gives rise to initial unstable electrical behaviour by cross-examination of samples with and without hydrogen either intrinsically or by thermal-assisted removal. Annealing conditions to remove hydrogen have been optimized on the basis of SIMS analyses and Gummel plot characteristics to control eventual degradation of the junctions. It has been found that under particular doping and growth conditions, C₂---H complexes can be formed. These defects appear more stable than C---H complexes which may explain the difficulty to remove hydrogen from some epitaxial layers.     

Analysis of the electrical characteristics of GaInP/GaAs HBTs including the recombination effect

An analytical model is used to predict the effects of surface recombination current on the gain and transit time of GaInP/GaAs heterojunction bipolar transistors (HBTs). The present analysis shows that consideration of the recombination current gives current gain values that are comparable to those of the experimental results. The dependence of current gain on temperature, base doping and emitter area are also analyzed, and the variation in collector current with emitter-base voltage, temperature and doping is considered.     

Surfactant effects of dopants on ordering in GaInP

The use of surfactants for control of specific aspects of the VPE growth process is beginning to be studied for both the elemental and III/V semiconductors. The objective is to change the characteristics of the material grown epitaxially by the addition of a surfactant during growth. Most reported surfactant effects for semiconductors relate to some detail of the morphology of the growing films. For ordered semiconductor alloys the effects can be much more dramatic, including major changes in the electrical and optical properties. Since the bandgap energy is dependent on the microscopic arrangement of the atoms in an alloy with a fixed composition, the change in order parameter induced by the surfactant translates into a marked change in the bandgap energy. This paper presents the results of a study of the effects of n-type (Te and Si), p-type (Zn), and isoelectronic (Sb) dopants on the ordering process in GaInP grown by OMVPE. All of the dopants studied were found to decrease or eliminate ordering; however, the mechanisms are quite different. The donor Te apparently affects the adatom attachment kinetics at steps on the (001) surface, a surfactant effect. On the other hand the donor Si was found to decrease the degree of order by an entirely different mechanism, attributed to an increase in the Ga and In diffusion coefficients in the bulk. It apparently does not involve the surface. Disordering due to the acceptor Zn was found to occur by the same mechanism. The isoelectronic impurity Sb is found to act as a surfactant and to decrease the order parameter by changing the surface reconstruction, eliminating the [110]-P dimers that provide the thermodynamic driving force for formation of the CuPt structure during growth.     

Determination of ordering induced birefringence in (Al)GaInP

In this paper, the first measurement of the ordering induced birefringence in GaInP and (Al_0.33Ga_0.67)InP is presented. It is found that these ordered crystals are positively birefringent far below the bandgap and become negatively birefringent at the bandgap. The method to measure the birefringence is based on the modifications introduced by ordering to the mode structure of planar waveguides. The change of the sign of birefringence at the bandgap is due to the highly anisotropic interband matrix element. The dispersion of the measured birefringence is in good agreement with six-band k·p calculations.     

Localized deposition of GaAs/GaInP heterostructures using LP-MOVPE

GaInP layers were grown selectively by low pressure MOVPE in patterned SiO₂ masks on GaAs (100) substrates. The variation of the composition and spontaneous ordering phenomena were analysed by Raman spectroscopy and photoluminescence. In contrast to GaInAs, the composition of GalnP shows only a very weak dependence on the size of the structures. On the other hand, there is a shift of the bandgap energy up to 40 meV with decreasing size of the stripes caused by ordering of the Ga and In atoms. Based on these findings lattice matched GaAs/GalnP multilayers were grown to delineate the growth history of the structures. It was demonstrated that the growth habit of deposition in narrow slits (>1μm) can be used to produce mesa-like stripes with dimensions below 100 nm on top of the mesa. Results of GaAs/GaInP quantum wells selectively grown on top of a mesa are presented.     

GaInP single junction and GaInP/GaAs two junction thin-film solar cell structures by epitaxial lift-off

The epitaxial lift-off (ELO) technique was used in forming a thin-film GaInP/GaAs two-junction monolithic tandem solar cell structure. First, the GaInP single junction solar cell to be used in the tandem cell structure as a top cell was thinned by the ELO process. Although the ELO process and the transfer to the quartz substrate caused a strain in the thin-film cell after separation from the GaAs substrate, the photoluminescence peak intensity was not decreased. This shows that defects, such as those causing carrier loss, were not introduced on the thin-film cell during the thinning process. The key issue for thin-film cell fabrication is to avoid damaging the AlInP window layer during the selective etching (HF etchant), by which the thin-film cell is released from the GaAs substrate. A GaInP/GaAs monolithic tandem structure was also thinned by the same process with a GaInP single junction cell. Characteristics of the single-junction GaInP cell and individual cells in the GaInP/GaAs tandem structure were examined. It was found that the spectral response remains almost the same as that for cells with a GaAs substrate, thus confirming the feasibility of using the ELO process to fabricate thin-film GaInP/GaAs cells.     

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.     

Mg掺杂AlInP 650 nm LD外延片的Zn扩散研究

利用石英闭管法,对Mg掺杂AlInP 650 nm LD外延片进行Zn扩散,分析了扩散温度和时间2个参数对Zn扩散的影响.采用光致发光(PL)谱和电化学蒸涂(ECV)方法研究了Zn扩散产生的影响.PL谱结果表明,Zn扩散引起了AlGaInP/GaInP多量子阱(MQW)有源层的组分无序,使PL谱的峰值蓝移,最大蓝移为54 nm,约175 meV.ECV测量结果显示,Zn已经扩散到MQW有源区,MQW区域的p型载流子浓度为4.4×1017 cm-3.     

Effect of Sb on the quantum efficiency of GaInP solar cells

The energy bandgap of GaInP solar cells can be tuned by modifying the degree of order of the alloy. In this study, we employed Sb to increase the energy bandgap of the GaInP and analyzed its impact on the performance of GaInP solar cells. An effective change in the cutoff wavelength of the external quantum efficiency of GaInP solar cells and an effective increase of 50 mV in the open‐circuit voltage of GaInP/Ga(In)As/Ge triple junction solar cells were obtained with the use of Sb. Copyright © 2016 John Wiley & Sons, Ltd.