Investigation of high quality GaAs:In layers grown by liquid phase epitaxy

Indium-doped GaAs layers are investigated by low-field Hall effect, photoluminescence, and double crystal x-ray diffraction in order to study the influence of the In concentration on the electrical, optical, and crystallographic properties. The layers were grown by liquid phase epitaxy from solution with In concentrations in the range 0–10 at.%. It was found that epitaxial growth from the melt with 7 at.% In content produces the highest quality epitaxial layers.     

Al-Ga-As-dopant phase-equilibria for liquid phase epitaxy

Calculated phase data for the Al-Ga-As-dopant liquid phase epitaxial system are presented. Modifications to interaction parameters are made. Firstly, the activity coefficient of the ternary solid phase has been taken as unity, i.e., solid solution AlGa-AlAs is assumed ideal, which permits the elimination of discontinuities when aluminum atomic fraction in the liquid goes to zero. Accordingly, the Al-As interaction parameter must be recalculated and a value of 2122-14T cal/mole (from 973 to 1273°K) has been obtained. With these changes in the interaction parameters, the model allows a fairly accurate fit to the solidus and liquidus data, and former problems associated with high temperatures and high and low aluminum atomic concentrations in the liquid have been resolved. Finally, the theory is extended to the case of a fourth component which acts as a dopant. Tin is taken as an example. Interaction parameters for this dopant have been calculated permitting a close adjustment to the experimental values. In order to achieve a more precise fitting an aluminum dependence in the Al-Sn interaction parameter is suggested.     

Properties of Bi-doped PbTe layers grown by liquid phase epitaxy under controlled Te vapor pressure

Effects of Bi doping in PbTe liquid-phase epitaxial layers grown by the temperature difference method under controlled vapor pressure (TDM-CVP) are investigated. For Bi concentrations in the solution, x_Bi, lower than 0.2 at.%, an excess deep-donor level (activation energy E_d≈0.03–0.04 eV) appears, and Hall mobility is low. In contrast, for x_Bi>0.2 at.%, Hall mobility becomes very high, while carrier concentration is in the range of 10¹⁷ cm⁻³. Inductive coupled plasma (ICP) emission analysis shows that, for x_Bi=1 at.%, Bi concentration in the epitaxial layer is as high as N_Bi=2.3–2.7 × 10¹⁹ cm⁻³. These results indicate that Bi behaves not only as a donor but also as an acceptor, and the nearest neighbor or very near donor-acceptor (D-A) pairs are formed, so that strong self-compensation of Bi takes place. Carrier concentration for highly Bi-doped layers shows a minimum at a Te vapor pressure of 2.2 × 10⁻⁵ torr for growth temperature 470°C, which is coincident with that of the undoped PbTe.     

Micropipe and dislocation density reduction in 6H-SiC and 4H-SiC structures grown by liquid phase epitaxy

We investigated silicon carbide (SiC) epitaxial layers grown by liquid phase epitaxy (LPE). The layers were grown on 6H-SiC and 4H-SiC well-oriented (0001) 35 mm diameter commercial wafers as well as on 6H-SiC Lely crystals. A few experiments were also done on off-axis 6H-SiC and 4H-SiC substrates. Layer thickness and growth rate ranged from 0.5 to 50 microns and 0.5 to 10 μm/h, respectively. Layers were investigated by x-ray diffraction, x-ray topography, and selective chemical etching in molten KOH. It was found that dislocation and micropipe density in LPE grown epitaxial layers were significantly reduced compared with the defect densities in the substrates.     

GaN grown by hydride vapor phase epitaxy on p-type 6H-SiC layers

6H-SiC/GaN pn-heterostructures were grown by subsequent epitaxial growth of p-SiC by low temperature liquid phase epitaxy (LTLPE) and n-GaN by hydride vapor phase epitaxy (HVPE). For the first time, p-type epitaxial layers grown on 6H-SiC wafers were used as substrates for GaN HVPE growth. The GaN layers exhibit high crystal quality which was determined by x-ray diffraction. The full width at a half maximum (FWHM) for the ω-scan rocking curve for (0002) GaN reflection was ∼120 arcsec. The photoluminescence spectra for these films were dominated by band-edge emission. The FWHM of the edge PL peak at 361 nm was about 5 nm (80K).     

Energy-Band offsets and electroluminescence in n-InAs1-xSb1-x/N-GaSb heterojunctions grown by liquid phase epitaxy

Isotype heterojunctions of n-InAsSb/N-GaSb were grown by liquid phase epitaxy from Sb-solution. The current-voltage and capacitance-voltage characteristics of these structures were investigated. Valence band and conduction band offsets were measured to be AEv = 0.40 eV and AEc = 0.84 eV, respectively, in good agreement with theory. Electroluminescence emission at 4 am from these isotype heterojunction diodes is reported for the first time. The injection mechanisms determining luminescence efficiency are also briefly discussed.     

Quantum-well Inp-Inl−xGaxPl−zAsz heterostructure lasers grown by liquid phase epitaxy (LPE)

By means of computer-controlled liquid-phase epitaxy (LPE), undoped single-and multiple-quantum-well InP-In_l-xGa_xP_l-zAs_z (x∼0.13, z∼0.29) heterostructures of uniform well (Lz∼150 ?) and coupling barrier size (200–300? ) have been grown and examined in photolumi-nescence. Undoped samples are used to eliminate problems with impurities and to make possible study of the fundamental properties of quaternary quantum-well heterostructures. For a single quaternary quantum-well heterostructure of small enough well size (Lz∼150?), which still collects holes but not electrons (electron scattering length >L_Z), hot-electron (p²/2m_n∼ΔE_c) recombination from E_c(InP) to bound holes in the valence-band well (i.e., to E_v(InGaPAs)), leads to stimulated emission in a band ∼ 80 meV below E_g(InP) and thus to an estimate of Δ E_v∼ 80 meV (ΔE_c∼2×ΔE_v) for the InP-InGaPAs valence-band discontinuity. On rectangular samples of multiple quantum-well LPE InP-In_1-xGa_xP_1-zAs_z heterostructures laser operation has been identified on LO phonon sidebands located below the lowest confined-particle transitions. A 5-barrier, 6-well quaternary quantum-well heterostructure exhibits phonon-assisted laser operation shifted in energy below confined-particle transitions by an InGaAs-like phonon (ˉhω_L0∼31 meV) and for an 11-barrier, 12-well heterostructure shifted by an InP-like phonon (ˉhω_L0∼43 meV). The 11+12 quaternary quantum-well heterostructure is shown to be capable of operating as a room temperature continuous (CW, 300K) laser (ˉhω<E_g ) .     

Electric current controlled liquid phase epitaxy of GaAs on N+ and semi-insulating substrates

Electric current controlled liquid phase epitaxy (LPE) of GaAs has been performed on both n⁺ and semi-insulating substrates. Growth is induced by current flow across the substrate-melt interface. The furnace temperature is held constant during growth so that direct electrical control of the growth process is achieved. The dependence of the growth rate on both the electric current density across the substrate-melt interface and the ambient furnace temperature was determined. Current densities from 5 to 20 A/cm² were employed and furnace temperatures ranging from 680 to 800°C were used. Sustained steady state growth rates as small as 0.022μm/min and as large as 1.4μm/min were obtained. For a given furnace temperature and current density, the measured growth rates on semi-insulating substrates range from 48% to 77% of the rates obtained on n⁺ n substrates. The surface morphology of the epitaxial layers is observed to depend on the electric current density employed during growth. Electric current controlled doping modulation was studied in epitaxial layers grown from unintentionally doped melts. The degree of doping modulation achieved is approximately proportional to the change in applied current density. Approximately a 40% increase in the net electron concentration is obtained by changing the current density from 10 to 30 A/cm² during growth. Preliminary experiments with tin doped epitaxial layers indicate that similar changes in the amount of tin incorporation can be achieved.     

Stoichiometry-dependent deep levels in undoped p-type Al0.38Ga0.62As grown by liquid phase epitaxy

Photocapacitance (PHCAP) and photoluminescence (PL) measurements were applied to unintentionally doped p-type Al_0.38Ga_0.62As grown by liquid phase epitaxy using the temperature difference method under controlled vapor pressure. PHCAP spectra revealed three dominant deep levels at E_v+0.9, E_v + 1.45, and E_v+1.96 eV, and a deep level at E_v+0.9−1.5 eV which was not neutralized by forward bias injection. These level densities increase with increasing arsenic vapor pressure and net shallow acceptor density. Furthermore, PL spectra reveal a deep level at 1.6–1.7 eV. The PL intensity of this deep level increases with increasing arsenic vapor pressure. These deep levels are thought to be associated with excess As.     

Methods for reducing deep level emissions from ZnSe grown by organometallic vapor phase epitaxy

Methods for reducing deep level emission from ZnSe layers grown by photo-assisted organometallic vapor phase epitaxy were studied using photo-luminescence. A number of approaches to achieve reductions in deep level emissions were investigated. One of them was the use of a flow modulation technique. Reduction in the deep level emissions was observed when the layers were annealed under zinc-rich conditions during this growth process. The effect of cadmium as an isoelectronic dopant in ZnSe was also studied. It was observed that “doping” levels of cadmium resulted in considerable reduction in deep level emissions from ZnSe layers. Layers were grown under different II/VI ratios, and compared to cadmium doped layers of similar ratios. Reduction in deep level emissions were observed in all cadmium doped layers. Cadmium, therefore, seems to be the most promising isovalent dopant for reducing the deep level emissions in ZnSe.     

Thin-layer liquid phase epitaxy of InGaPAs heterostructures in short intervals (< 100 ms): Non-diffusion-limited crystal growth

Data are presented showing that two different mechanisms control the LPE growth of InGaPAs in the step-cooled technique. An automated growth apparatus, which allows an accurate and reproducible selection of growth times as short as ~9ms, is used to study the thickness and the growth rate of InGaPAs layers as a function of growth time for times ranging from ~9ms to ~20s. For long intervals the measured InGaPAs epilayer thickness is shown to vary as the square root of the growth time, as expected for the case of diffusion-limited growth. When the growth period is reduced to < 200ms , the quaternary layer thickness is greater than the diffusion-controlled value and, in addition, is practically independent of the growth time. Auger depth profile data on InGaPAs layers grown from ~ 9ms to ~ 120ms are presented showing that layers are uniform in composition. Photoluminescence data on InGaPAs layers grown under non-diffusion limited conditions are shown to be different in composition than the relatively thick layers grown under diffusion-limited conditions, at longer times, from melts with the same liquidus compositions. Data are presented indicating the existence of both of these distinct compositions in a single ~ 800 Å InGaPAs layer grown in ~ls. It is shown that thick InGaPAs layers of uniform composition can be grown, by the step-cooled LPE process, by stacking a number of thin layers grown in short intervals. Data are presented indicating that thin-layer stacks can be used to improve the performance characteristics of heterostructure lasers.     

Growth and characterization of InAsSb layers on GaSb substrates by liquid phase epitaxy

High quality InAs_1−xSb_x semiconductor films were successfully grown on (100) GaSb single crystal substrates using liquid phase epitaxy technique (LPE). The crystalline structure and lattice mismatch between film and substrate were investigated by high-resolution X-ray diffraction (HRXRD). The surface roughness and the interface morphology of the epitaxial film-on-substrate were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and optical microscopy. These results show the high-purity InAs_1−xSb_x epitaxial layers with mirror-like surface and rms ranges from 0.5 to 2 nm, and a sharp interface between substrate and ternary film. The optical properties of the layers were studied by low temperature photoluminescence (PL) spectroscopy. PL spectrum of the ternary film shows one radiative emission peak with narrow full width at half-maximum, which is an evidence of the good crystalline quality of the epilayer. It is worth to mention that the InAsSb films were grown on GaSb substrates for compositions of Sb with x=0.16 without introducing any intermediate composition buffer layer between the GaSb substrate and the film as reported in previous works.     

液相外延生长GaAs微探尖刻蚀剥离技术研究

介绍一种用于扫描近场光学显微术(NSOM)传感头的GaAs微探尖的生长剥离技术.通过SiO2掩膜窗口,利用一次选择液相外延制备周期性阵列的GaA微探尖.在GaAs衬底与GaAs微探尖之间引入AlGaAs层,并对窗口大小的AlGaAs层进行选择腐蚀,将单个GaAs微探尖从GaAs衬底上剥离下来.扫描电子显微镜显示的结果表明,此微探尖具有金字塔结构、表面光滑凡转移过程无损伤.这种技术制备的GaAs微探尖的形貌与质量主要由晶体的结构决定,具有可重复,表面光滑、适合批量生产的优点.     

Fabrication and electrical characterization of n-InSb on porous Si heterojunctions prepared by liquid phase epitaxy

Thin films of InSb were grown on p-type porous silicon (PSi) (1 1 1) substrates by liquid phase epitaxy (LPE) to obtain monocrystalline InSb epilayer on a PSi substrate for low cost device applications. The structural characterization of the devices was carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The X-ray diffraction measurements indicate that InSb monocrystalline epilayer was successfully grown onto PSi. The current-voltage (I-V) characteristics of n-InSb/p-PSi heterojunction devices were measured in the temperature range of 298-398 K. The measurements indicate that these heterojunctions have good rectifying characteristics. The estimated zero-bias barrier height φ_BO and the ideality factor η show strong temperature dependence. The conventional Richardson plot exhibits linear behavior in the entire temperature range indicating that the conduction seems to be predominantly due to thermionic emission mechanism. In addition, the capacitance-voltage characteristics are investigated at frequency of 1 MHz. The built-in potential of the heterojunction is determined after eliminating the effect of the capacitance effect of the interface state caused by the lattice mismatch.     

Liquid phase epitaxy of pbtese lattice-matched to pbsnte

The conditions for liquid phase epitaxial growth of lead telluride selenide on lead tin telluride are presented, and some physical and electrical properties of the epilayers grown are described. The composition and energy gap of the quaternary lead salt compound, lead tin telluride selenide, lattice-matched to lead selenide and to lead-tin telluiride, as determined from published data, is also presented. The growth of PbSnTeSe on PbSnTe is demonstrated.     

Internal strain and dislocations in Ga1−xAs crystals grown by liquid phase epitaxy/electroepitaxy

Strain relaxed, low dislocation density In_xGa_1−xAs crystals, 0 < x <0.2, have been successfully grown by liquid phase electroepitaxy on the GaAs substrate, despite the crystal/substrate lattice mismatch. Residual strain in these novel substrates is below 10⁻⁴, at least an order of magnitude lower than in the molecular beam epitaxially (MBE) or metalorganic chemical vapor deposition-grown ternary buffer layers of similar composition. Threading dislocation density induced by both the crystal/substrate lattice mismatch and unavoidable composition variations has been reduced from the low 10⁶ cm⁻² range, while growing directly on GaAs, to the mid 10⁴ cm^-2 by employing both the MBE grown ternary buffer layer and selective lateral overgrowth of an SiO₂ mask which, prior to the crystal growth, was deposited on the buffer layer and patterned by photolithography with 10 μm wide, oxide free seeding windows. The full width at half maximum of the rocking curves measured for In_xGa_1−xAs crystals grown by liquid phase epitaxy/electroepitaxy on patterned, closely lattice matched buffer layers was in the 20–23 arc s range. Further reduction of the dislocation density and a more uniform dislocation distribution is expected by modifying the initial growth conditions, improving substrate preparation, and optimizing the seeding window geometry.     

Electrical and structural characterization of GaAs vertical-sidewall epilayers grown by atomic layer epitaxy

Electrical and structural measurements have been performed on novel test structures incorporatingp-type GaAs epilayers grown by organometallic vapor phase atomic layer epitaxy on the vertical sidewalls of semi-insulating GaAs rods formed by ion-beam-assisted etching. Preliminary results indicate that the vertical-sidewall epilayers have excellent crystal quality and sufficient electrical quality to support a sidewall-epitaxy device technology. Some examples of candidate electronic, electrooptic, and photonic devices for vertical-sidewall fabrication are FETs, resistors, waveguides, modulators, and quantum-wire and quantum-dot lasers.     

ZnMgSSe/ZnSSe/ZnSe-heterostructures grown by metalorganic vapor phase epitaxy

ZnMgSSe heterostructures have been grown in a low-pressure metalorganic vapor phase epitaxy system with the precursors dimethylzinc triethylamine, ditertiarybutylselenide, tertiarybutylthiol, and biscyclopentadienylmagnesium at 330°C and a total pressure of 400 hPa. The optimization of the single layers was carried out by means of low temperature photoluminescence. Only the near band edge emission was observable with negligible deep levels. The heterostructures consisting of a triple ZnSe quantum well showed intense luminescence which hints at an effective carrier confinement. Scanning transmission electron microscopy investigations of the heterostructures still showed structural detects since the layers were not lattice matched to the GaAs substrate yet.     

Deep level transient spectroscopy assessment of silicon contamination in AlGaAs layers grown by metalorganic vapor phase epitaxy

A systematic silicon contamination has been detected by deep level transient spectroscopy in undoped and n-type doped (Te, Se, Sn) AlGaAs layers, grown in two different metalorganic vapor phase epitaxy reactors. DX center generation by substitutional donors, with very specific capture and emission thermal barriers (fingerprints), is the key to unambiguously identifying their presence, with detection limits well below the standard secondary ion mass spectroscopy capability. We comment on the potential sources of Si contamination (most common in this epitaxial technique), and on the relevance of such contamination to interpreting correctly experimental data related to the microscopic structure of DX centers.     

The preparation of CdSnP2/InP heterojunctions by liquid phase epitaxy from Sn-solution

Heterojunctions of n-type CdSnP₂ on p-type InP substrates are prepared by LPE from Sn-solutions. The significance of the tipping arrangement, of the initial solution composition and of the temperature program for preparing LPE layers of high quality is discussed. Heterodiodes cleaved from such LPE wafers electroluminesce under forward bias between 1 μm and 1.8 %m with internal quantum efficiencies approaching 1.8% at room temperature. When used as photovoltaic detectors, the quantum efficiency of these diodes surpasses that of Si for λ > 1.09 μ, and exceeds 1% for all wave-lengths between 0.96 and 1.3 μ.