Valence band offsets in strained GaAs1−xPx/GaAs heterojunctions

Valence band offsets at [100]-oriented heterojunctions between tensile-strained GaASj_1−xP_x and unstrained GaAs are studied experimentally and theoretically. Light-hole (LH) and heavy-hole (HH) offsets are first extracted from the well-width dependence of valence subband splittings observed in luminescence spectra of tensile-strained GaAs_1−xP_x/GaAs quantum wells of various compositions (x = 0.06,0.09, and 0.19). This data is then combined with results from two other laboratories, yielding a set of 30 independent experimental offset values for junctions with compositions throughout the range 0.06≤x ≤0.32. The data are found to be highly consistent, with linear fits δE_LH = −140x (meV) and δE_HH= −401x (meV) describing the measured offsets to within less than 5 meV on average. Experimental results are then compared with theoretical predictions for the GaAs_1−x P_x/GaAs system obtained from a tight-binding model for strained heterojunctions. Predictions from the tight-binding calculations are found to lie within experimental scatter for the LH offsets, which define the valence band edge in these heterostructures, while magnitudes of the tight-binding HH offsets exceed measured values by ~20% on average.     

Process parameter dependence of impurity-free interdiffusion in GaAs/AlxGa1−xAs and InxGa1−yAs/GaAs multiple quantum wells

The dependence of the impurity-free interdiffusion process on the properties of the dielectric cap layer has been studied, for both unstrained GaAs/Al_xGa_1−xAs and pseudomorphic In_y Ga_1−yAs/GaAs MQW structures grown by molecular beam epitaxy. The influence of the cap layer thickness, composition, and deposition technique on the degree of interdiffusion were all systematically investigated. Electron-beam evaporated SiO₂ films of varying thickness, chemical-vapor-deposited SiO_xN_y films of varying composition, and spin-on SiO₂ films were used as cap layers during rapid thermal annealing (850-950°C). Photoluminescence at 10K has been employed to determine the interdiffusioninduced bandgap shifts and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with the cap layer thickness (e-beam SiO₂) up to a limit determined by saturation of the outdiffused Ga concentration in the SiO₂ caps. A maximum concentration of [Ga] = 4–7 ×10¹⁹ cm⁻³ in the SiO₂ caps was determined using secondary ion mass spectroscopy profiling. Larger band-edge shifts are also obtained when the oxygen content of SiO_xN_y cap layers is increased, although the differences are not sufficiently large for a laterally selective interdiffusion process based on variations in cap layer composition alone. Much larger differences are obtained by using different deposition techniques for the cap layers, indicating that the porosity of the cap layer is a much more important parameter than the film composition for the realization of a laterally selective interdiffusion process. For the calculated In_0.2Ga_0.8As/GaAs interdiffusion coefficients, activation energies E_A and prefactors D_o were estimated to ranging from 3.04 to 4.74 eV and 5 × Kh⁻³ to 2 × 10⁵ cm²/s, respectively, dependent on the cap layer deposition technique and the depth of the MQW from the sample surface.     

Critical thickness of GaAs/InGaAs and AlGaAs/GaAsP strained quantum wells grown by organometallic chemical vapor deposition

In this paper we describe a study of strained quantum wells (QWs) as a means to experimentally observe the critical thickness (h _c) for the formation of interfacial misfit dislocations. Two material systems were investigated: GaAs/In_0.11Ga_0.89As, in which the QW layers are under biaxialcompression, and Al_0.35Ga_0.65As/GaAs_0.82P_0.18, in which the QW layers are under biaxialtension. Samples were grown by atmospheric pressure organometallic chemical vapor deposition, and characterized by low-temperature photoluminescence (PL), x-ray diffraction, optical microscopy, and Hall measurements. For both material systems, the observed onset of dislocation formation agrees well with the force-balance model assuming a double-kink mechanism. However, overall results indicate that the relaxation is inhomogeneous. Annealing at 800–850° C had no significant effect on the PL spectra, signifying that even layers that have exceededh _c and have undergone partial relaxation are thermodynamically stable against further dislocation propagation.     

Photoresponse study of normal incidence detection in p-type GaAs/AIGaAs multiple quantum wells

We studied p-type GaAs/AIGaAs multiple quantum well (MQW) materials as a possible alternative to the current n-type GaAs/AIGaAs MQWs for infrared detection. The advantage of p-type MQWs is that absorption of infrared radiation at normal incidence is not selection rule forbidden as it is for the n-type. We have verified that significant photoresponse occurs at normal incidence in p-type MQWs. We studied changes in the photoresponse spectrum as a function of well width and temperature. The MQW heterostructures were designed to use bound to continuum intersubband absorption in the GaAs valence band and to have a peak photoresponse near 8 μm. The photoresponse spectrum was compared to the first theoretical model of the bound to continuum absorption in p-type GaAs/ AlGaAs MQWs. The theoretical absorption curve was found to be in good qualitative agreement with the experimental results.     

Contactless electroreflectance study of a GaAIAs/lnGaAs/ GaAs/GaAIAs step quantum well structure

Using contactless electroreflectance at 300 and 77K, we have studied the inter-subband transitions from a GaAlAs/InGaAs/GaAs/GaALAs step quantum well structure (small well inside a large well) consisting of two layers A (In_xGa_1−xAs) and B (GaAs) with widths L_A and L_B, respectively, bounded by two thick barrier regions of Ga_x Al_yAs. By comparison of the observed spectral features with an envelope function calculation, including the effects of strain, we have been able to characterize the potential profile of the structure, i.e., L_A, L_B, x, and y. There is very good agreement between experiment and the intended materials param-eters. Such configurations are of considerable importance since (a) they form the basis for pseudomorphic high electron mobility transistors, and (b) also have applications in optoelectronics due to their large Stark shifts.     

AlGaAs/GaAs多量子阱红外探测器暗电流特性

介绍了AlGaAs/GaAs多量子阱红外探测器(QWIP)暗电流与噪声的关系和降低暗电流的途径:基于湿法化学腐蚀工艺制作了300μm×300μm台面单元器件,并用变温液氦杜瓦测试系统在不同温度下对红外探测器暗电流进行了测试并分析.在温度小于40 K时,随着温度的改变暗电流没有明显的变化;当温度大于40 K时,暗电流随着温度的升高迅速变大,正、负偏压下QWIP暗电流具有不对称特性.     

Wavelength tuning in strained layer InGaAs-GaAs-AlGaAs quantum well lasers by selective-area MOCVD

Selective-area growth and regrowth using conventional atmospheric pressure metalorganic chemical vapor deposition is investigated for wavelength tuning in strained layer In_xGa_1-xAsGaAs-Al_y Ga_1-yAs quantum well lasers. Growth inhibition from a silicon dioxide mask is the mechanism used for the selective-area growth rate enhancement. By varying the width of the oxide stripe opening, differences in the growth rate yield different quantum well thicknesses, and hence different lasing wavelengths for devices on the same wafer. Both two-and three-step growth processes are utilized for selective-area epitaxy of strained layer In_xGa_1-xAs-GaAs quantum well active regions, with lasers successfully fabricated from the three-step growth. Scanning electron microscopy and transmission electron microscopy indicate that the absence of an oxide mask during Al_yGa_1-yAs growth is essential for successful device operation. A wide wavelength tuning range of over 630Å is achieved for lasers grown on the same substrate.     

Interdiffusion and relaxation in metalorganic vapor phase epitaxy grown InGaAs/GaAs strained layer quantum wells

Low pressure metalorganic vapor phase epitaxy grown strained InGaAs/GaAs quantum well structures have been characterized by photoluminescence and x-ray diffraction. It is shown that beyond the pseudomorphic limit, these structures show considerable gallium/indium interdiffusion at the interfaces and partial strain relaxation in the quantum well layers.     

Growth and characterization of (111)B InGaAs/GaAs multi-quantum well PIN diode structures

High quality piezoelectric strained InGaAs/GaAs multi-quantum well structures on (111)B GaAs substrates have been grown by solid-source molecular beam epitaxy in a PIN configuration. 10K photoluminescence (PL) shows narrow peaks with widths as low as 3 meV for a 25-period structure while room temperature (RT) PL shows several higher order peaks, normally forbidden, indicating breaking of inversion symmetry by the piezoelectric field. Furthermore, both the 10K PL peak position and the form of the RT PL spectra depend on the number of quantum wells within the intrinsic region, suggesting that the electric-field distribution is altered thereby. Diodes fabricated from these structures had sharp avalanche breakdown voltages (V_bd) and leakage currents as low as 8 × 10⁻⁶ A/cm² at 0.95 V_bd, indicating quality as high as in (100) devices. On leave from: Department de Ingeniera Electronica, Universidad Politecnica de Madrid, Madrid, Spain.     

The effects of laser irradiation on InGaAs/GaAs multiple quantum wells grown by metalorganic molecular beam epitaxy

We studied the effects of Ar ion laser irradiation during the growth of InGaAs/ GaAs multiple quantum wells (MQW) structures by metalorganic molecular beam epitaxy. Structural and optical properties were characterized by Nomarski microscopy, Dektak stylus profiler, and low-temperature photoluminescence (PL) measurements. For MQW structures grown at a relatively low substrate temperature (500°C), the laser irradiation influences greatly the growth process of the In^Ga^^As well and results in a large blue shift of about 2000à in the PL peak. Such a large blue shift suggests that laser modification during growth could have some novel applications in optoelectronics. On the other hand, the laser irradiation has relatively small effects on samples grown at a higher substrate temperature (550°C).     

1.3 μm InGaAs/GaAs strained quantum well lasers with InGaPcladding layer

Using MOVPE, we fabricated strained quantum well 1.3 μm lasers with an InGaP cladding layer on a GaAs substrate. The lasers had a high gain coefficient of 60 cm^-1. Lasers with high reflection facets had a low threshold current density of 500 A/cm², and a high characteristic temperature of 100 K     

Investigation on the emission wavelength of GaInNAs/GaAs strained compressive quantum wells on GaAs substrates

In this paper, we study the effect of the incorporation of nitrogen in strained GaInAs quantum well structures. We evaluate the influence of nitrogen on the conduction band energy by using the band anticrossing model. The incorporation of nitrogen appears to decrease the bandgap energy and increase the emission wavelength. The reduction of energy is due to the interaction of the energy of the conduction band with the level of nitrogen and more the concentration of nitrogen increases, more the energy of the band gap decreases. On the other hand, the emission wavelength increases, the advantage of the incorporation of nitrogen in such structures is to vary the wavelength between 0.980 and 1.3 μm while exploiting of course the composition of gallium, composition of nitrogen and the thickness of the quantum well. Less temperature insensitive devices are so intended to be fabricated.     

A1.3 μm strained quantum well laser on a graded InGaAs buffer with a GaAs substrate

We grew a 1.3 μm strained-layer quantum well (SL-QW) laser with InGaP cladding layers on a lattice-relaxation buffer layer by metalorganic vapor phase epitaxy. For the lattice-relaxation buffer, we used a compositionally graded InGaAs/GaAs structure. The significantly reduced surface roughness of the InGaP cladding layers achieved by supplying a large amount of H₂Se enables CW operation of our 1.3 μm SL-QW laser. We achieved a low threshold current of less than 10 mA and a high characteristic temperature of 100K around room temperature.     

A revised ideal model for AlGaAs/GaAs quantum well solar cells

Quantum well solar cells (QWSCs) are heterostructure devices intended to achieve higher efficiencies than conventional cells. This paper extends a previous model for QWSC current-voltage characteristics by revising the equations for the absorbed flux and by introducing expressions to calculate radiative recombination coefficients and well effective densities-of-states. This revised model is in agreement with previous experimental results for AlGaAs/GaAs. Since the revised model incorporates detailed balance calculations, its predictions are consistent with the efficiency restrictions of this theory. The revised model, however, does predict efficiency enhancements for QWSCs in some configurations if non-radiative recombination is dominant, even in such a poor QWSC material as AlGaAs/GaAs.     

Formation of GaAsP interface layers monitored by reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) has been used to study As-by-P exchange during metalorganic vapor phase epitaxy. The study focuses on the processes occurring during switching from GaAs to GaInP, especially the effect of purging PH₃ over a GaAs surface. GaAsP/GaAs superlattices of different periodicity were grown and the P-content was determined by high-resolution x-ray diffraction and correlated to the RAS spectra. From the temperature dependence of the P-content, an activation energy of 0.56 eV was estimated for the incorporation mechanism. In addition to the insights into the processes at mixed group-V heterointerfaces, our study demonstrates the reproducibility of RAS transients that thus can be used for process monitoring.     

Investigation of the band structure of the strained systems InGaAs/GaAs and InGaAs/AIGaAs by high-pressure photoluminescence

In_xGa_1-xAs quantum wells grown pseudomorphically in GaAs and AlGaAs with values ofx up to 0.25 have been studied by photoluminescence under high hydrostatic pressure. We concentrate here on the pressure range where the emissions quench and take on the characteristics of theX-minima. In the InGaAs/GaAs structures, these transitions display an unexpected pressure coefficient, -2.6 meV/kbar, twice that of theX minima in GaAs. We assign these transitions to theX minima in the wells, and therefore make a direct measurement of the strainedX positions as a function of composition. In the InGaAs/AIGaAs structures the crossovers occur against theX-minima in the barriers and these crossovers yield an accurate value for the band offset ratio for InGaAs/GaAs heterojunctions which is found to be 60:40 (CB:VB).     

Realization of in-situ sub two-dimensional quantum structures by strained layer growth phenomena in the InxGa1- xAs/GaAs system

We confirm that as the misfit strain in pseudomorphic epitaxial layer increases, surface thermodynamics controlled growth modes can change from a layer-by-layer to a three-dimensional (3-D) island mode. Both in-situ reflection high energy electron diffraction studies and in-situ scanning tunneling microscopy studies are utilized to demonstrate this transition to 3-D growth. This concept allows one to grow GaAs/In_xGa_1-xAs/GaAs heterostructures where the electrons in In_xGa_1-xAs are possibly confined in lower dimensions.     

Growth of InAIGaAs strained quantum well structures for reliable 0.8 μm lasers

Incorporation of indium into the quantum well materials of graded-index separate confinement heterostructure quantum well lasers has proven to be a key to imparting a much needed robustness to such lasers. By growing wells which contain both indium and aluminum along with gallium, operating wavelengths can be engineered to fall in the technologically important range of 0.8 microns, appropriate for pumping Nd:YAG. The organometallic vapor phase epitaxial growth of these strained-layer structures faces extra challenges rooted in the competing influences on the energies of the quantized states. At a minimum, meeting wavelength targets requires achieving control of the quaternary composition and of the quantum well thickness. Because laser elements are relatively large, lateral uniformity of wavelength is a critical issue. Device performance is influenced by basic material quality, which is a function of such fundamental growth parameters as temperature, V/III ratio, and growth rate. We have grown InAIGaAs structures using various combinations of growth conditions and well composition and thickness combinations, and evaluated and life-tested lasers in CW mode. The reactor’s performance in achieving composition and thickness uniformity is reported, as are data on the influence of the effects of growth conditions on device performance.     

Diode lasers emitting at 1190 nm with a highly strained GaInAs quantum well and GaAsP compensating layers MOCVD-grown on a GaAs substrate

Laser heterostructures with an active region consisting of a highly strained GaInAs quantum well located between GaAsP compensating layers were grown by metal-organic chemical vapor deposition on GaAs substrates. Stripe mesa-structure laser diodes of 100 μm aperture emitting at 1190 nm were fabricated. The highest emission power of these diodes in the CW mode amounted to 4.5 W per output mirror. Due to the presence of compensating GaAsP barriers, the GaInAs quantum well remains unrelaxed, which eliminates the spread in the maximum emission power of laser diodes produced from the same heterostructure.     

Diode lasers emitting at 1220 nm with a highly strained GaInAs quantum well and GaAsP compensating layers MOCVD-grown on a GaAs substrate

Heterostructures with an active region containing a Ga_0.59In_0.41As quantum well located between GaAs_{1 − y} P _y compensating layers were studied using photoluminescence spectroscopy. It was shown that an increase in the phosphorus content in compensating layers makes it possible to obtain unrelaxed heterostructures with wider Ga_0.59In_0.41As quantum wells. On the basis of photoluminescence studies, the parameters of such a composite active region were chosen with a view to attaining the longest lasing wavelength possible. Laser heterostructures with a composite active region consisting of a highly strained Ga_0.59In_0.41As quantum well located between GaAs_0.85P_0.15 compensating layers were grown on GaAs substrates by metalloorganic chemical vapor deposition. Stripe mesa-structure laser diodes of 100-μm aperture emitting at 1220 nm were fabricated. The highest emission power of these laser diodes in the continuous-wave regime amounted to 2 W per output mirror.