Low-pressure Metalorganic Vapor Phase Epitaxy Research Articles

High-speed growth of thick high-purity β-Ga2O3 layers by low-pressure hot-wall metalorganic vapor phase epitaxy

High-speed growth of thick, high-purity β-Ga2O3 homoepitaxial layers on (010) β-Ga2O3 substrates by low-pressure hot-wall metalorganic vapor phase epitaxy was investigated using trimethylgallium (TMGa) as the Ga precursor. When the reactor pressure was 2.4–3.4 kPa, the growth temperature was 1000 °C, and a high input VI/III (O/Ga) ratio was used, the growth rate of β-Ga2O3 could be increased linearly by increasing the TMGa supply rate. A thick layer was grown at a growth rate of 16.2 μm h−1 without twinning. Incorporated impurities were not detected, irrespective of the growth rate, demonstrating the promising nature of β-Ga2O3 growth using TMGa.

Low-Pressure MOVPE: The time of the pioneers

This paper is the author’s personal view on the development of low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) which took place in the late 70s and early 80s of last century. Pioneering work has been performed by the author and his co-workers in the labs of Thomson CSF, France, leading to many sophisticated devices including GaAs-GaAlAs- and InP-InGaAsP-based quantum well laser diodes which could be driven in CW mode, being grown successfully the first time by LP-MOVPE. This was only possible by a thorough analysis and a deep understanding of the underlying processes, in particular to overcome basic hurdles like the comparably low purity of the available MOVPE precursors at that time. Many results have kept their importance over decades and are still of major significance.

Realizing high injection current density up to 200 kA-cm−2 in electrically pumped AlGaN ultraviolet laser diodes on c-Sapphire substrate

Previously, we attempted to the optically-pumped AlGaN ultraviolet (UV) laser diodes (LDs) on AlN template on c-Sapphire substrate, using low-cost epitaxial growth technique in low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE). As a result, stimulated lasing spectra at 274 nm were observed for the first time at room temperature (RT) in the optically pumped LD. However, the electrically-pumped AlGaN-based (270–298 nm)-band UV LD was quite challenging due to both the issue of low injection current density and light confinement factor (Г). In this paper, the injection current density in the electrically pumped AlGaN UV LD at 283 nm emission was remarkably improved up to 200 kA-cm−2 by using undoped Al-graded cladding and Al-graded Mg-doped p-AlGaN hole source layer (HSL). Also, some weak oscillation at 296 nm near to the main spontaneous emission peak of 298 nm-band LD under the 600 mA (24 V) current using pulse width of 50 ns at RT was observed. To further improve the lasing phenomenon, some theoretical studies was conducted, by making the p-side waveguide (p-WG) layer non-doped (Mg = 0) and also increasing the quantum-well (QW) thickness from 2 nm to 4 nm in the multiquantum-wells (MQWs). Consequently, the optical-loss was significantly reduced from 22.7 to 8.7 cm−1 and the Г was enhanced from 1.77% (Experimental LD) to 4.65% (simulated LD).

Highly Transparent p‐AlGaN‐Based (326–341 nm)‐Band Ultraviolet‐A Light‐Emitting Diodes on AlN Templates: Recent Advances and Perspectives

The epitaxial growth of transparent p‐AlGaN‐based ultraviolet‐A (UVA), light‐emitting diodes (LEDs) may solve the problems of UVA light absorption through the GaN buffers and p‐GaN contact layers at (326–341 nm)‐band emission, respectively. Herein, first, an idea of conventional n‐AlGaN buffer layer (BL) and n‐AlGaN electron source layer (ESL) for the suppression of threading dislocation density (TDD) and enhancement of internal quantum efficiency (IQE) of UVA emitters, using low‐pressure metalorganic vapor‐phase epitaxy (LP‐MOVPE) is attempted. As a result, the total‐TDDs is reduced from ≈ 3 × 109 cm−2 to ≈ 1 × 109 cm−2 in the n‐AlGaN ESL of a 326 nm‐band UVA multiquantum‐wells (MQWs), and IQE is also improved from 30% to 52% at room temperature (RT). Second, an idea of Si‐doped n‐AlGaN Superlattices (SLs)‐based BL, using LP‐MOVPE is challenged. Subsequently, a record IQE of 56% at RT and high crystal quality in 341 nm‐band UVA MQWs are observed. Finally, using a well thickness ≈ 2 nm in SLs‐based UVA MQWs, the light power and external quantum efficiency (EQE), respectively, are remarkably enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm‐Band UVA LED. The perspective for the improvements of UVA emitter's performances is also discussed.

Leakage mechanism in Al x Ga1−x N/GaN heterostructures with AlN interlayer

Leakage of Al x Ga1−x N/GaN heterostructures was investigated by admittance–voltage profiling. Nominally undoped structures were grown by low-pressure metal-organic vapor-phase epitaxy. The investigated structures had an Al-content of 30%. They are compared to structures with an additional 1 nm thick AlN interlayer placed before the Al0.3Ga0.7N layer growth, originally to improve device performance. Conductance of field effect transistor devices with AlN interlayer, measured from depletion of the two-dimensional electron gas (2DEG) to zero volt bias at frequencies ranging from 50 Hz to 10 kHz, could be described by free charge carriers using a Drude model. The voltage dependent conductance shows a behavior described by either Poole-Frenkel emission or Schottky emission (SE). From the size of the conductance, as well as simulation of the tunneling current injected from the gate under off-state conditions by universal Schottky tunneling, SE is obvious. Evaluating the data by SE, we can locate the leakage path, of tens of nm in the range between gate and drain/source with contact to the 2DEG, originating from the AlN interlayer. The static dielectric constant in growth direction, necessary for the evaluation, is determined from various Al x Ga1−x N/GaN heterostructures to ϵ ||(0) = 10.7 ± 0.1.

Room‐temperature laser operation of a (Ga,In)As/Ga(As,Bi)/(Ga,In)As W‐type laser diode

The ongoing pursuit for laser device emitting in the near-infrared spectral region on GaAs substrates has led to various material systems and device concepts. Alloys containing dilute amounts of Bismuth are promising candidates due to the already substantial band gap shift when incorporating low molar fractions of Bi in the GaAs host lattice. However, devices emitting at technologically essential wavelengths of 1.3 and 1.55 μm have yet to be demonstrated using this material system. Especially the non-equilibrium nature of the growth conditions required to grow the metastable material makes epitaxial growth with high molar fractions of Bi challenging. An alternate approach to reach the desired wavelengths exploits a type-II band alignment between two materials to push the emission wavelength further into the telecom bands. Here, room-temperature laser operation of the first type-II structure employing Ga(As,Bi) as hole confining layer and (Ga,In)As as electron confining layer is demonstrated. Sample growth is conducted by low-pressure metalorganic vapour phase epitaxy. Broad area laser devices are processed and characterized by electroluminescence measurements. A threshold current density of 3.86 kA/cm2 and emission wavelength of 1037 nm are observed, showing this device concept's potential for future lasers in the telecom bands.

Effects of dissipative electron tunneling manifested in the photocurrent of a GaAs p-i-n photodiode with a double InAs quantum dot layer

We report on the results of experimental studies of the photocurrent (PC) of photodiodes based on GaAs p-i-n structures with InAs/GaAs(001) double asymmetric quantum dot (DAQD) arrays obtained by self-assembling in the process of low-pressure metal-organic vapor phase epitaxy (LP-MOVPE). Three peaks were observed in the dependence of the PC on the reverse bias, measured under photoexcitation with a photon energy equal to the energy of the interband ground state transition in larger InAs QDs. These peaks were attributed to photoexcitation of electrons from the ground hole states in larger QDs into the ground electron states followed by resonant dissipative (with absorption or emission of optical phonons) and conservative tunneling into the GaAs conduction band via the ground electron states in smaller QDs. The PC dependence on the bias voltage agrees qualitatively with the theoretical field dependence of the probability of 1D dissipative tunneling between the QDs.

Early stages of InP nanostructure formation on AlInAs

The potential of ${\mathrm{Al}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{As}/\mathrm{InP}$ as a flexible candidate for future device applications relies on a thorough understanding of the growth mechanisms of this system during metalorganic vapor-phase epitaxy (MOVPE). We present a systematic study of the morphology of InP films grown on macroscopically lattice-matched ${\mathrm{Al}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{As}$ during low-pressure MOVPE. InP and AlInAs alloys are often assumed to be fully compatible, as long as macroscopic lattice matching is preserved. However, thin InP films show a variety of spontaneously formed nanostructures as the growth conditions are varied. We have examined the systematic variation of individual growth conditions (temperature, V/III ratios, growth rate, and misalignment angle and direction) to obtain a comprehensive picture of the morphological evolution of this system. The analysis of these experiments is based on a kinetic model of the deposition and decomposition of polyatomic precursors and the subsequent adatom kinetics, which is used to form characteristic lengths from combinations of kinetic coefficients associated with each process. The orderings of these lengths for particular ranges of growth conditions account for many aspects of the morphological evolution on perfectly oriented and misoriented surfaces. For perfectly oriented surfaces, differences in surface free energies are suggested as the driving force for Volmer-Weber growth and the observed formation of multilayer features. Finally, we examine the transition from quantum rings to quantum dots during postgrowth recovery when the group-V source is switched from phosphine to arsine. A qualitative explanation of similar observations in InAs/InP based on As/P exchange is consistent with our findings.

The influence of quantum well and barrier thicknesses on photoluminescence spectra of InGaAs/AlInAs superlattices grown by LP-MOVPE

In the presented work, the influence of the quantum well and barrier thicknesses on optical characteristics of InGaAs/AlInAs superlattices was reported. Six different structures of In0.53Ga0.47As/Al0.48In0.52As superlattices lattice-matched to InP were grown by low pressure metal organic vapour phase epitaxy (LP-MOVPE). Optical properties of the obtained structures were examined by means of photoluminescence spectroscopy. This technique allows quick, simple and non-destructive measurements of radiative optical transitions in different semiconductor heterostructures. The analysis of recorded photoluminescence spectra revealed the influence of the quantum well and barrier thicknesses on the emission line energy.

Growth of ZnMgSeTe nearly lattice-matched to ZnTe and p-type doping by low-pressure MOVPE

Zn1-xMgxSeyTe1-y layers have been grown on (100) ZnTe substrates by low-pressure metalorganic vapor phase epitaxy using dimethylzinc, bis-methylcyclopentadienyl magnesium ((MeCp)2Mg), diethylselenide and diethyltelluride as source materials. The Mg mole fraction of Zn1-xMgxSeyTe1-y layer can be controlled successfully by varying (MeCp)2Mg transport rate. P-type doping of this material has been tried using tris-dimethylaminophosphorus as a dopant source. The influence of annealing temperature or dopant transport rate upon the electrical property has been investigated. All the layers show p-type conduction even without annealing. Through this study, a maximum carrier concentration of 2.5×1018cm−3 is obtained.

Photoluminescence and electrical properties of P-doped ZnTe layers grown by low pressure MOVPE

Photoluminescence (PL) spectra and electrical properties of both as-grown and annealed phosphorus-doped ZnTe layers grown by low-pressure metalorganic vapor phase epitaxy have been clarified as a function of reactor pressure or dopant transport rate. The carrier concentration of as-grown layer increases with decreasing reactor pressure or increasing dopant transport rate, in good agreement with PL behavior. Similar tendency is also found for annealed layer. Annealing treatment induces the enhancement of the carrier concentration, independent of reactor pressure or dopant transport rate. Through this study, a high carrier concentration near 1019cm−3 has been attainable.

Structural and optical properties of vanadium ion-implanted GaN

The field of advanced electronic and optical devices searches for a new generation of transistors and lasers. The practical development of these novel devices depends on the availability of materials with the appropriate magnetic and optical properties, which is strongly connected to the internal morphology and the structural properties of the prepared doped structures. In this contribution, we present the characterisation of V ion-doped GaN epitaxial layers. GaN layers, oriented along the (0001) crystallographic direction, grown by low-pressure metal-organic vapour-phase epitaxy (MOVPE) on c-plane sapphire substrates were implanted with 400keV V+ ions at fluences of 5×1015 and 5×1016cm−2. Elemental depth profiling was accomplished by Rutherford Backscattering Spectrometry (RBS) and Secondary Ion Mass Spectrometry (SIMS) to obtain precise information about the dopant distribution. Structural investigations are needed to understand the influence of defect distribution on the crystal-matrix recovery and the desired structural and optical properties. The structural properties of the ion-implanted layers were characterised by RBS-channelling and Raman spectroscopy to get a comprehensive insight into the structural modification of implanted GaN and to study the influence of subsequent annealing on the crystalline matrix reconstruction. Photoluminescence measurement was carried out to check the optical properties of the prepared structures.

Effects of the substrate misorientation on the structural and optoelectronic characteristics of tensile GaInP quantum well laser diode wafer

Three tensile GaInP quantum well laser diode wafers were grown in the same run by low-pressure metalorganic vapor phase epitaxy on GaAs substrates misoriented by 10°, 15°, and 22° toward (1 1 1)A respectively. The photoluminescence peak wavelength and intensity of the active region, the strain mismatch and the doping carrier concentration of the epitaxial layers, the whole thickness and the surface morphology of the wafers were all found to be strongly dependent on the misorientation of the substrates. Considering the comprehensive effects of material, electrical, and optical properties, the 15° substrate was superior to the 10° substrate for the growth of shorter wavelength red-light laser diode wafer, while substrate with too large misorientation of 22° was not a good choice for the epitaxy of red-light laser diode wafer.

Technology and properties of low-pressure metalorganic vapour phase epitaxy grown InGaAs/AlInAs superlattice for quantum cascade laser applications

Technology and properties of low-pressure metalorganic vapour phase epitaxy grown InGaAs/AlInAs superlattice for quantum cascade laser applications

Admittance–voltage profiling of AlxGa1−xN/GaN heterostructures: Frequency dependence of capacitance and conductance

Admittance–voltage profiling of AlxGa1−xN/GaN heterostructures was used to determine the frequency dependent capacitance and conductance of FET devices in the frequency range from 50 Hz to 1 MHz. The nominally undoped low pressure metal-organic vapor-phase epitaxy structures were grown with an Al-content of 30%. An additional 1 nm thick AlN interlayer was placed in one structure before the Al0.3Ga0.7N layer growth. For frequencies below 108 Hz it is convenient to use equivalent circuits to represent electric or dielectric properties of a material, a method widely used, for example, in impedance spectroscopy. We want to emphasize the relation between frequency dependent admittance–voltage profiling and the corresponding equivalent circuits to the complex dielectric function. Debye and Drude models are used for the description of the frequency dependent admittance profiles in a range of depletion onset of the two-dimensional electron gas. Capacitance- and conductance-frequency profiles are fitted in the entire measured range by combining both models. Based on our results, we see contributions to the two-dimensional electron gas for our samples from surface states (80%) as well as from background doping in the Al0.3Ga0.7N barriers (20%). The specific resistance of the layers below the gate is above 105 Ω cm for both samples and increases with increasing negative bias, i.e., the layers below the gate are essentially depleted. We propose that the resistance due to free charge carriers, determined by the Drude model, is located between gate and drain and, because of the AlN interlayer, the resistance is lowered by a factor of about 30 if compared to the sample without an AlN layer.

The structural and optical properties of metal ion-implanted GaN

The practical development of novel optoelectronic materials with appropriate optical properties is strongly connected to the structural properties of the prepared doped structures. We present GaN layers oriented along the (0001) crystallographic direction that have been grown by low-pressure metal–organic vapour-phase epitaxy (MOVPE) on sapphire substrates implanted with 200keV Co+, Fe+ and Ni+ ions. The structural properties of the ion-implanted layers have been characterised by RBS-channelling and Raman spectroscopy to obtain a comprehensive insight into the structural modification of implanted GaN layers and to study the subsequent influence of annealing on crystalline-matrix recovery. Photoluminescence was measured to control the desired optical properties. The post-implantation annealing induced the structural recovery of the modified GaN layer depending on the introduced disorder level, e.g. depending on the ion implantation fluence, which was followed by structural characterisation and by the study of the surface morphology by AFM.

Low-frequency noise in reverse-biased Schottky barriers on InAlN/AlN/GaN heterostructures

We present low-frequency gate noise characteristics of InAlN/AlN/GaN heterostructures grown by low-pressure metal-organic vapor phase epitaxy. The electric field in the InAlN barrier is determined from C-V measurements and is used for gate leakage current modeling. The latter is dominated by Poole-Frenkel emission at low reverse bias and Fowler-Nordheim tunneling at high electric field. Several useful physical parameters are extracted from a gate leakage model including polarizations-induced field. The gate noise fluctuations are dominated by trapping-detrapping processes including discrete traps and two continuums of traps with distributed time constants. Burst noise with several levels and time constant values is also observed in these structures. Low-frequency noise measurements confirm the presence of field-assisted emission from trap states. The 1/f noise model of McWorther is used to explain the 1/f-like noise behavior in a restricted frequency range.

Optimisation of a carbon doped buffer layer for AlGaN/GaN HEMT devices

This work reports on the optimisation of carbon doping GaN buffer layer (BL) for AlGaN/GaN HEMT (high electron mobility transistor) structures, grown by low pressure metal–organic vapour phase epitaxy (LP-MOVPE) on 3in. SiC semi-insulating substrates. The incorporation of carbon impurities in GaN is studied as a function of the growth conditions, without using an external carbon source. We observed that the C incorporation can be effectively controlled over more than one order of magnitude by tuning the reactor pressure and the growth temperature, without degradation of the crystalline properties of the GaN layers.HEMT structures with a specific barrier design were grown with different carbon dopings in the GaN BL and processed into transistors to evaluate the impact of the BL doping on the device performances. A significant improvement of the HEMT drain leakage current and of the breakdown voltage was obtained by increasing the carbon incorporation in the GaN BL. The RF performances of the devices show a trade-off between leakage currents and trapping phenomena which are enhanced by the use of carbon doping, limiting the delivered output power. An output power as high as 6.5W/mm with a Power Added Efficiency of 70% has been achieved at 2GHz by the HEMT structures with the lowest carbon doping in the BL.

Analysis of the core–shell interface between zinc-blende GaP and wurtzite ZnO

Core–shell GaP/ZnO nanowires (NWs) were prepared in a two-step process: (1) GaP NWs were grown on GaP substrate by low-pressure metalorganic vapour phase epitaxy using 30nm Au seeds as nucleation centres, (2) the GaP NWs were covered in a thin nanocrystalline Ga-doped ZnO layer by sputtering in a Perkin Elmer planar RF diode system. Electrical contacts were processed to individual GaP/ZnO NWs using electron beam lithography, evaporation and lift-off of metallic layers: Au/Zn (GaP core) and Au/Al (ZnO shell). Electrical and photocurrent measurement of the NWs confirmed that a radial pn heterojunction was formed between the GaP core and ZnO shell.

Capacitance properties and simulation of the AlGaN/GaN Schottky heterostructure

Abstract The paper presents the results of capacitance measurements on GaN/AlGaN/GaN Schottky heterostructures grown on an Al 2 O 3 substrate by Low-Pressure Metal–Organic Vapour-Phase Epitaxy (LP-MOVPE). Dependences of the capacitance–voltage (CV) characteristics on the frequency of the measuring signal allow analysing the properties of the 2D electron gas (2DEG) at the AlGaN/GaN heterojunction. Exact location of the hetero-interface below the surface (20 nm) was determined from the concentration profile. Temperature variations of the CV curves reveal the influence of bulk defects in GaN and of the traps at the AlGaN/GaN interface. Electrical activity of these defects was characterized by capacitance Deep Level Transient Fourier Spectroscopy (DLTFS). Experimental results of CV measurements were supported by simulating the properties of the GaN/Al 0.2 GaN 0.8 /GaN Schottky heterostructure in dependence on the influence of the concentration of donor-like traps in GaN and of the temperature upon the CV curves.