Background Carrier Research Articles

MOVPE Growth of High Electron Mobility AlGaN/GaN Heterostructures

ABSTRACTWe have fabricated AlxGa1−xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm2/Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5×1012 cm−2. The undoped AlxGa1−xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH4. The effect of intentional n-type doping of the AlxGa1−xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped AlxGa1−xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100Å thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped AlxGa1−xN. These initial results demonstrate that the electrical properties of AlxGa1−xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.

Metalorganic molecular beam epitaxial growth of InP and InGaP with tertiarybutylphosphine for the application of carbon-doped base heterojunction bipolar transistors

Metalorganic molecular beam epitaxy (MOMBE) of InP and In 0.5Ga 0.5P has been studied by using the elemental group III sources (In and Ga) and tertiarybutylphosphine (TBP) as phosphorus source. The catalysis of heated tantalum (Ta) is important to effectively decompose the TBP. Undoped In 0.5Ga 0.5P with n-type background carrier concentration of n = 4 × 10 15 cm -3 was obtained, and the controllability of Si doping in In 0.5Ga 0.5P ranging from 9.7 × 10 17 to 1.3 × 10 19 cm -3 was shown by using cracked Si 2H 6. In the growth of InP with the TBP, carbon was unintentionally incorporated as donor and was well controlled in the range suitable for InP/In 0.53Ga 0.47As HBTs by the growth temperature and the flow rate of TBP. The MOMBE growth of In 0.5Ga 0.5P and InP layers was applied to fabricate carbon-doped base In 0.5Ga 0.5P/GaAs and InP/In 0.53Ga 0.47As HBTs, respectively. A common emitter current gain h fe of 26 at a collector current density J C of 1.4 kA/cm 2 was obtained for In 0.5Ga 0.5P/GaAs HBTs with a hole concentration as high as 1 × 10 20 cm -3 in the base. Carbon-doped base InP/In 0.53Ga 0.47As HBTs were also fabricated by applying the carbon-doping technique for the InP emitter and a h fe value of 16 at J C = 36 A/cm 2 was obtained for the first device.

Metalorganic chemical vapor deposition growth of InN for InN/Si tandem solar cell

A new tanden solar cell having an InN top cell is proposed since InN is a binary material having a bandgap of about 1.9 eV suitable for a top cell in a 2-junction tandem cell. An estimation of the conversion efficiency of InN/Si tandem cell has been made. For a 2-terminal cell, the bandgap of InN, 1.95 eV, is close to the optimum for AM 0 illumination because currents between top and bottom cells can be matched. The MOCVD growth of InN has been studied as a key technology for device fabrication. Epitaxial growth of InN directly on Si using the MOCVD technique is unsuccessful because of the formation of an amorphous SiNx layer on the Si substrate surface. Characterization has been made for single-crystalline InN films grown on α-Al2O3 substrates. Compared with InN films grown at a reduced pressure (≈ 76 Torr), those grown at the atmospheric pressure have a lower background carrier concentration and a higher electron mobility, although thier surface morphology and crystallographic quality are somewhat worse.

Te doping of GaAs using diethyl-tellurium

The performance of the gaseous precursor diethyl-tellurium (DETe) for n-type doping of GaAs is studied. We report on both the properties of the molecular source DETe as well as on the suitability of the dopant element tellurium. Te is found to be an excellent dopant, comparable to Si in the lower-doping regime and superior at highest-doping levels. DETe reveals itself to be a convenient and reproducible doping source. At substrate temperatures above 540 °C, dopant desorption (most probably Te) is observed which limits its applications. Memory effects after doping up to the mid 1018 cm−3 range can be eliminated. At still higher-doping levels an optimized technique limits unintentional background carrier concentrations in subsequent layers to the 1014 cm−3 range.

Determination of metal cations by capillary electrophoresis effect of background carrier and completing agents

The determination of trace metals in aqueous samples can be readily accomplished by capillary electrophoresis (CE) via indirect absorbance detection. Methods for simultaneously determining alkali, alkaline earth and transition metal ions and Group IB, IIB and IVA metals ions were developed. Imidazole, benzylamine, ephedrine or pyridine was used as the carrier buffer and the background absorbance provider. Glycolic acid, α-hydroxy-isobutyric acid or succinic acid was used as the complexing agent. The elements determined were Li, Na, K, Cs, Mg, Ca, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Ag, Al and Pb. All ions could be separated in less than 15 min. In most instances reported here, more than a dozen ions could be separated in 5–10 min. All peaks were well separated and baseline resolved ( i.e., no peaks overlapped), except Ag and Al, which required a separate and additional analysis. The detection limit was in the range 0.02 (Na)−208 ppb (Cr) with the electrokinetic injection mode (10 kV, 5 s). The reproducibility was 1% for the migration time and better than 5% for the peak height for most metal ions. The calibration graphs were linear for most ions in the concentration range 10 −5−10 −3 M ( R 2 = 0.9995−0.9999) using the hydrodynamic injection mode. Concentrations lower than 10 −5 M can be determined using the electrokinetic injection mode, but the calibration graph is not linear. The methods developed here are well suited for determining metal ions in a variety of real samples.

High quality In0.52Al0.48As grown by modulated arsenic molecular beam epitaxy

A modulated arsenic molecular beam epitaxy method has been developed for growing high quality In0.52Al0.48As under a condition compatible to the optimal growth condition of In0.53Ga0.47As without using a buffer layer or growth interruption. In this method, the As shutter is periodically opened and closed while the Al and In shutters are held constantly open. By adjusting the As shutter modulation rate, a layer-by-layer growth mode can be maintained throughout the growth as evident from the persistent strong reflection high-energy electron diffraction intensity oscillations. Without an In0.53Ga0.47As buffer layer, the unintentionally doped In0.52Al0.48As samples grown at 500 °C show very strong photoluminescence intensity and n-type conductivity with a background carrier concentration of ∼5×1015 cm−3 and electron mobilities of 1900 and 3900 cm2/V s at 300 and 77 K, respectively. Very narrow 77 K photoluminescence spectra have been observed from In0.53Ga0.47As/In0.52Al0.48As quantum well stack structures grown by this new method.

Chemical beam epitaxy of high purity InP using tertiarybutylphosphine and 1,2 bis-phosphinoethane

This paper compares tertiarybutylphosphine (TBP), and 1,2 bis-phosphinoethane (BPE) as phosphorus precursors for the growth of InP in chemical beam epitaxy. The thermal decomposition of both sources in a low pressure cracking cell produces dimer P 2 and tetramer P 4 species. The P 2 to P 4 ratio is a function of the cracker temperature θ c , the dimer P 2 is the dominant species for θ c = 800 ° C . High quality InP layers, showing good surface morphology, were grown using low values of the V to III flux ratio R V/III , leading to a very low consumption of the group V product. The lower R V/III value ( R V/III ⋍ 3 ) is obtained with the BPE molecule, which contains two phosphorus atoms. 77 K Hall mobility values of 62 500 cm 2/Vs and 26 500 cm 2/Vs were obtained using TBP and BPE respectively and triethylindium as group III source. With both phosphorus sources, background carrier density levels as low as N D − N A < 10 15 cm −3 are obtained.

Growth of Undoped Indium Phosphide by OMVPE in an Inverted‐Vertical Reactor Using Trimethylindium and Tertiarybutylphosphine and Phosphine

The as‐grown morphologies and background carrier concentrations of undoped epilayers grown at 650°C were determined as a function of phosphorus source [tertiarybutylphosphine (TBP) or phosphine ] and V:III ratio in an inverted‐vertical (IV) metallorganic chemical vapor deposition (MOCVD) reactor. Specular surface morphology was obtained over the entire growth surface (16.6 cm2) at a minimum to trimethylindium (TMIn) ratio of 10, and at a minimum TBP to TMIn ratio of 33. Below these V:III ratios, the area of the epilayer surfaces exhibiting specular morphology decreased as the V:III ratio was reduced; however, the layer thicknesses remained uniform. All undoped epilayers were n‐type. The carrier concentration obtained with in the specular area of the epilayers was on the order of at V:III ratios up to ca. 20; at a V:III ratio of 40, decreased to 1012 cm−3. The carrier concentration obtained with TBP in the specular area of the epilayers was about at V:III ratios up to 33. The relation of V:III ratio to morphology and the distribution of visible phosphorus deposition on the reactor tube walls during growth indicated that the decomposition characteristics of and TBP are considerably different in the inverted vertical reactor than in other system configurations. The decomposition characteristics observed here are empirically correlated with decomposition mechanism unique to the IV geometry.

Metalorganic Molecular Beam Epitaxy of AlGaAs Using APAH

To take advantage of the benefits of gaseous precursors for the growth of AlGaAs, a reduction of the carbon and oxygen uptake from the sources is necessary. Therefore, new precursors have been developed recently to overcome these problems. An entirely new approach is the use of an intramolecular saturated precursor, where the coordinative saturation against oxygen is realised by means of a double ring structure. 1-(3-dimethylaminopropyl)-1-ala-cyclohexan (APAH) is a representative of this class of precursors. In metalorganic molecular beam epitaxy (MOMBE) the suitability of APAH for the growth of AlxGa1-xAs layers (0≤x≤1) was investigated in combination with TEGa and arsine. For the grown layers morphology and crystallinity are found to be excellent, whereas the background carrier concentrations are in the range of p=1017 to 1019 cm-3. SIMS measurements clearly identify carbon as the main acceptor. Other than carbon, nitrogen is also incorporated from APAH. Despite this, APAH is useful for particular heterostructure applications as demonstrated by quantum well structures with excellent interfaces. Therefore, in MOMBE of AlGaAs, APAH is a suitable Al precursor for applications where p-type doping of 1017 cm-3 or above is required or acceptable (e.g., in HBT).

Electrical properties and deep levels of InAlAs layers grown by metalorganic chemical vapor deposition

Deep donors whose thermal activation energy is about 250 meV are detected in undoped InAlAs layers grown by MOCVD. These deep donors determine the background carrier concentrations and cause a discrepancy between the donor concentrations measured by capacitance-voltage measurements and free carrier concentrations measured by Hall measurements. Oxygen as high as 1019 cm-3 is detected in these InAlAs layers. Oxygen atoms are thought to be the probable candidates of the deep donors. Oxygen in InAlAs layers is also thought to form deep levels, whose activation energies are 0.5 eV (level A), 0.3 eV (level B), 0.07 eV (level C), and 0.45 eV (level D). The reduction of oxygen in InAlAs layers results in low concentrations of both deep donors and deep levels.

Symmetric self-electro-optic effect device array grown by metalorganic vapor phase epitaxy using GaAs/Al0.04Ga0.96As shallow quantum wells

A self-electro-optic effect device (SEED) epistructure grown by low-pressure metalorganic vapor phase epitaxy (MOVPE) is demonstrated. The symmetric SEED using GaAs/Al0.04Ga0.96As shallow quantum well (SQW) exhibits a contrast ratio of 2.9 at 5 V bias with 50 pairs of GaAs wells. This high contrast results from the low background carrier concentration of absorption region which is obtained by the temperature controlled compensation of the acceptors and donors. The absorption data suggest that the MOVPE technique is comparable to the MBE technique in growing the high performance SQW epistructure.

Tertiaybutyldimethylantimony for GaSb growth

Antimony-containing alloys have the lowest energy bandgaps of the conventional III-V semiconductors. Organometallic vapor phase epitaxy (OMVPE) has proven to be an effective and convenient technique for the production of the antimony alloys, normally using trimethylantimony (TMSb) as the antimony source. However, TMSb has several problems: (a) it decomposes at relatively high temperatures due to the strong CH3-Sb bond, and (b) its use results in carbon contamination from the active methyl radicals, especially for the growth of aluminum-containing alloys. Recently, tertiarybutyldimethylantimony (TBDMSb) has been successfully used to grow high quality InSb epitaxial layers at temperatures as low as 325°C with low V:III ratios. This paper reports the use of TBDMSb to grow GaSb layers at temperatures from 500° to 650°C. Group V:III ratios close to unity are required for good surface morphologies. The optimum V:III ratio decreases slightly as the growth temperature is lowered due to the incomplete decomposition of trimethylgallium (TMGa). The growth efficiency is about 1 × 104 μm/mole, indicating that there are no significant parasitic reactions between TMGa and TBDMSb. The as-grown layers are p-type. The background carrier concentration is nearly independent of growth temperature. Low temperature (10K) photoluminescence (PL) intensities are also nearly independent of temperature in this range. The PL spectra consist of two peaks, a bound exciton peak at about 792 meV and a native acceptor complex (VGa, GaSb) peak at about 775 meV. At every growth temperature, the ratio of the PL intensity of the bound exiton peak to the native acceptor peak is always higher for samples grown using optimum conditions. The results show that TBDMSb can be used for the OMVPE growth of antimony-containing materials over a wide range of growth temperature.

Application of an electrochemical arsine generator on a high throughput MOVPE reactor

Quality GaAs buffer layers have been grown in a high throughput MOVPE reactor using a novel electrochemical arsine generator which reduces the required quantity of arsine gas on site to less than 0.1 lbs. Hall, secondary ion mass spectrometry (SIMS), photoluminescence (PL), and deep level transient spectroscopy (DLTS) measurements reveal material properties comparable to those obtained with high purity tank arsine. Background carrier concentrations in the low 10 14 cm −3 range were observed. Al x Ga 1− x As layers grown using generator arsine were shown by SIMS to have the same oxygen content as layers grown previously in this reactor using tank arsine but are higher than reported elsewhere. The oxygen contamination of Al x Ga 1− x As was observed to decrease with the addition of an arsine purifier. Preliminary observations on the reproducibility of generator arsine are presented.

Growth of InP in chemical beam epitaxy with high purity tertiarybutylphosphine

InP layers were grown by chemical beam epitaxy (CBE) using high purity thermally precracked tertiarybutylphosphine (TBP) and trimethylindium (TMI) as the source of the group III element. For optimized substrate temperature and V/III ratio, InP films of good electrical and optical quality have been obtained; the n-type background carrier concentration is (1–2) × 10 15 cm -3, with a Hall mobility at 77 K being μ 77 = 45,000 cm 2 V -1 s -1. Given the low value of the V/III ratio, and according to mass spectrosc measurements, the phosphorus species giving rise to epitaxy is expected to be the dimer P 2. The TBP consumption in CBE is very low when compared to organometallic vapour phase epitaxy (OMVPE), typicaly below 0.25 g/μm of InP layer.

Characteristics of GaAs, AlGaAs, and InGaAs materials grown by metalorganic chemical vapor deposition using an on-demand hydride gas generator

We report results on the properties of GaAs, AlGaAs, and InGaAs materials grown using a new, on-demand hydride gas generator. Low pressure arsine gas is generated from an arsenic containing precursor (KAsH2) by the controlled addition of water as a chemical activator. Both generated and bottled arsine are used to grow GaAs, AlGaAs, and InGaAs structures using atmospheric pressure metalorganic chemical vapor deposition. Using generated arsine, GaAs layers with background carrier concentrations of less than n=3×1013 cm−3 were produced across a growth temperature range of 625–725 °C using a V/III ratio of 30. InGaAs grown at 640 °C with V/III=30 exhibits a background carrier concentration of n=2.5×1014 cm−3 and mobility values of μ300 K=11 350 cm2/V s and μ77 K=71 200 cm2/V s. Photoluminescence measurements show highly resolved exciton spectra using either generated or bottled arsine with donor-bound exciton linewidths as narrow as 0.16 meV full width at half-maximum. Broad area GaAs/AlGaAs laser devices exhibit threshold current densities as low as 195 A/cm2. The results obtained from bulk layer, quantum well structure, and broad area laser device characterization indicate that the quality of materials produced using generated arsine is equivalent or superior to that of materials produced using a high quality bottled arsine source.

Metalorganic vapor phase epitaxical growth and 1.5-μm laser fabrication using ethyldimethylindium, tertiarybutylphosphine, and tertiarybutylarsine

High-quality InGaAsP lattice matched to InP was grown by low-pressure metalorganic vapor phase epitaxy using ethyldimethylindium (EDMIn), tertiarybutylphosphine (TBP), and tertiarybutylarsine (TBA). The background carrier concentrations of undoped InP and InGaAs were as low as 1.5×1015 and 2.2×1015 cm−3, respectively. Good compositional control of In-GaAsP was also possible. The performance of the double heterostructure wafers employed as lasers as characterized by threshold current density, internal loss, and internal quantum efficiency. The threshold current density for a 300-μm cavity was as low as 2.0 kA/cm2. In addition, ridge-waveguide distributed feedback lasers were successfully fabricated. These results show that EDMIn, TBP, and TBA might be used to replace conventional sources for growing InGaAsP/InP lasers.

Chemical beam epitaxy growth of high-quality InAlAs using TMAA

InAlAs has been grown by chemical beam epitaxy using trimethylamine alane for use in InAlAs/InGaAs and InAlAs/InP high‐electron‐mobility transistors (HEMTs). Oxygen levels as low as 6 × 1017 cm−3 have been achieved. Background carrier concentrations are on the order of 2 × 1014 cm−3, with a best 14‐K photoluminescence full width at half‐maximum of 18.5 meV. InAlAs/InGaAs submicrometer HEMTs exhibit transconductances as high as 890 mS/mm with a corresponding ft of 150 GHz.

Improved control of composition and electrical properties of liquid phase epitaxial (CdHg)Te layers

The use of powdered HgTe as a source of mercury in a graphite sliding boat used for liquid phase epitaxial (LPE) growth of Cd x Hg 1− x Te alloys from Te-rich solutions at 460°C has been found to give excellent reproducibility of alloy composition, x and thickness due to the low Hg loss rates during growth (typically 0.3 mg min -1). Reproducibility of the wavelength corresponding to an absorption coefficient, α, of 500 cm -1 ( λ α = 500 ) is ± 0.15 μm, while the thickness reproducibility is typically ± 1 μm for 20μm thick layers. The variation of composition with depth in the layers is typically (2–4)x10 -4 in x per μm. Background carrier concentrations in undoped layers after Hg-rich annealing to remove Hg vacancies are n = (6–8) x10 13 cm -3. Using SIMS analysis we have shown that In is 100% electrically active after Hg-rich annealing and that the In dopant does not diffuse during the anneal treatment. Copper (Cu) doping has been studied for p-type material in the range 8×10 15 to 2×10 18 cm -3. The Cu appears to be nearly 100% electrically active in as-grown layers, but during Hg-rich isothermal annealing at 250° C, a certain proportion of the Cu diffuses to the surface leaving only about 30% of the Cu in the bulk of the layer still electrically active.

Reduced incorporation of unintentional impurities and intrinsic defects in ZnSe and ZnS grown by MOVPE

In this paper we investigate the incorporation as well as the origin of unintentional impurities and intrinsic defects in ZnSe and ZnS grown by MOVPE. In ZnSe grown under optimized conditions on GaAs, we observe high electron mobilities ( μ 300 K = 370 cm 2/ V· s) and low background carrier concentrations ( n = 7 × 10 14 cm -3). The 10 K photoluminescence (PL) spectrum shows the I 2 emission and free excitons. Changes in the growth parameters cause the appearance of donor-acceptor pair recombinations at 2.72 eV. The Y-line (2.55 eV) is not observed in samples grown at temperatures below 420°C but is generated by high temperature treatment. This emission is therefore correlated to extended lattice defects such as dislocations created by strain. The so-called “Cu-green emission” (2.25−2.45 eV) not present in as-grown samples can be generated by a high temperature treatment in H 2, Zn or Se atmosphere in the MOVPE reactor. From these experiments we conclude that this defect is not only due to an extrinsic impurity. ZnS grown on GaAs and ZnS substrates is dominated by free exciton recombinations. PL emission related to a free exciton bound to extrinsic donors or to an acceptor is measured. In the PL spectra of similar ZnS layers on GaAs substrate additional emissions appear. Thus this emission is related to Ga and As impurities from the substrate. The emission wavelength of the SA centre is dependent on the substrate material.

Growth and Properties of Semi‐Insulating InP Using Multi‐Frit Trichloride Vapor Phase Epitaxy (MTVPE)

Semi‐insulating layers have been deposited using multi‐frit trichloride vapor phase epitaxy (MTVPE) for the first time. Iron doping was achieved using generated in situ by passing over pure iron foil (99.9999%) in the source zone. The background carrier concentration for undoped was . The resistivity of the semi‐insulating epitaxial layers was evaluated by current‐voltage measurements. Values as high as were measured, and current blocking was effectively maintained up to a bias of 20 V with a 3 μm thick semi‐insulating layer. An activation energy of 0.59 eV was estimated from the temperature dependence of the resistivity. Such layers have potential device applications for electrical isolation and particularly for ion‐implanted devices. Ion implantations were performed on the present layers and the results are compared with those obtained for MOCVD‐grown semi‐insulating InP material and for semi‐insulating substrates.