ALBERT

Notes: Two schemes of nucleation and growth of gallium nitride on Si(111) substrates are investigated and the structural and electrical properties of the resulting films are reported. Gallium nitride films grown using a 10–500 nm-thick AlN buffer layer deposited at high temperature (∼1050 °C) are found to be under 260–530 MPa of tensile stress and exhibit cracking, the origin of which is discussed. The threading dislocation density in these films increases with increasing AlN thickness, covering a range of 1.1 to 〉5.8×109 cm−2. Films grown using a thick, AlN-to-GaN graded buffer layer are found to be under compressive stress and are completely crack free. Heterojunction field effect transistors fabricated on such films result in well-defined saturation and pinch-off behavior with a saturated current of ∼525 mA/mm and a transconductance of ∼100 mS/mm in dc operation. © 2001 American Institute of Physics.

Notes: The various mechanisms responsible for the strain relaxation of strain-compensated GaInP/InAsP multilayers grown on InP(001) using low-pressure organometallic vapor-phase epitaxy (LP-OMVPE) were investigated using a combination of transmission electron microscopy (TEM), high-resolution x-ray diffraction (HRXRD), and reciprocal lattice mapping. We examined separately the effect of the misfit strain f as well as the total strain energy εT on the strain relaxation mechanisms. We also investigated the effect of the growth temperature Ts on roughening. For the structures composed of a small number of superlattice periods, N=10, TEM and HRXRD indicate that strain relaxation occurs essentially through non-homogeneities at the interfaces for increasing misfit strain f values (at least up to |f|=1%, the largest strain used in these experiments). In comparison, when the magnitude of the misfit strain is kept constant, increasing the number of periods eventually leads to a massive generation of dislocations in the multilayer. For |f|=0.75%, coherency breakdown was observed around the 14th–15th period in a 50-period sample. However, the strain-compensated multilayer structures can be in a metastable state since all layers are perfectly flat and no dislocations are visible in a 20-period sample with the same misfit strains in the layers. Finally, we observed that the growth temperature Ts had a drastic effect on the morphology of the layers: increasing Ts from 620 to 680 °C while keeping all other growth parameters constant introduced large periodic lateral thickness modulations as well as dislocation clusters in the structures. Diffraction contrast analysis in plan-view TEM indicates significant anisotropy with the features elongated in the [11¯0] direction. These results could be used as guidelines for the design of highly perfect and reliable device structures grown by LP-OMVPE. © 1997 American Institute of Physics.

Notes: The emission mechanisms of bulk GaN and InGaN quantum wells (QWs) were studied by comparing their optical properties as a function of threading dislocation (TD) density, which was controlled by lateral epitaxial overgrowth. Slightly improved excitonic photoluminescence (PL) intensity was recognized by reducing TD density from 1010 cm−2 to less than 106 cm−2. However, the major PL decay time was independent of the TD density, but was rather sensitive to the interface quality or material purity. These results suggest that TDs simply reduce the net volume of light-emitting area. This effect is less pronounced in InGaN QWs where carriers are effectively localized at certain quantum disk size potential minima to form quantized excitons before being trapped in nonradiative pathways, resulting in a slow decay time. The absence of any change in the optical properties due to reduction of TD density suggested that the effective band gap fluctuation in InGaN QWs is not related to TDs. © 1999 American Institute of Physics.

Notes: Extended defect reduction in GaN grown by lateral epitaxial overgrowth (LEO) on large-area SiO2/GaN/Al2O3 wafers by low pressure metalorganic chemical vapor deposition is characterized using transmission electron microscopy and atomic force microscopy. The laterally overgrown GaN (LEO GaN) has a rectangular cross section with smooth (0001) and {112¯0} facets. The density of mixed-character and pure edge threading dislocations in the LEO GaN (〈5×106 cm−2) is reduced by at least 3–4 orders of magnitude from that of bulk GaN (∼1010 cm−2). A small number of edge dislocations with line directions parallel to the basal plane are generated between the bulk-like overgrown GaN and the LEO GaN regions as well as at the intersection of adjacent merging LEO GaN stripes. The edge dislocations are most likely generated to accommodate the small misorientation between bulk-like GaN and LEO GaN regions as well as between adjacent single-crystal LEO GaN stripes. © 1998 American Institute of Physics.

Notes: An analysis and discussion of the device physics for the quantum-confined Stark effect based on barrier height and band alignment considerations is presented. It identifies two important design principles for band structure engineering of the multi-quantum well stack: (1) Due to the counterbalance relationship between field-induced redshift and field-induced polarization of the quantum well eigenstates, design strategies must look to attain an optimal balance or compromise between a minimum drive field and maximum absorption coefficient change. This can be achieved with an appropriate choice of the valence band discontinuity. (2) In III–V semiconductors, the strong asymmetry in the field response of the conduction and valence band eigenstates is due directly to the asymmetry of the conduction and valence band effective masses. As a result, optimum device performance is obtained by using a heterostructure with a disproportionately large conduction band offset to compensate the effective mass asymmetry and balance the field-induced wave function leakage in the conduction band to that in the valence band. The relative wave function leakage between conduction and valence bands is compared by examining tunneling currents through the quantum well barriers as a function of the electric field and barrier height. For conduction and valence band effective masses of, respectively, 0.055 and 0.5 times the free electron mass, the optimal band alignment requires a conduction band discontinuity 3–9 times greater than the valence band discontinuity. Applying these design principles for high speed, low drive voltage optical modulators shows that the overall performance of these devices may be improved by using a combination of balanced band alignments and low valence band barriers. The low valence band barriers reduce the drive field required to operate the devices, which has direct effects upon the drive voltage, device capacitance, attenuation coefficient, and optical coupling and propagation losses. The analysis and discussion is supported by experimental modulation depth and drive field data obtained from strained-layer multiple quantum well InAsP/InP and strain-compensated InAsP/InGaP optical modulators fabricated with layers grown on InP(001) by metalorganic vapor phase epitaxy. © 1998 American Institute of Physics.

Notes: The effect of dislocations on the electrical characteristics of GaN p-n junctions has been examined through current–voltage measurements. Lateral epitaxial overgrowth (LEO) was used to produce areas of low dislocation density in close proximity to areas with the high dislocation density typical for growth on sapphire. A comparison of p-n diodes fabricated in each region reveals that reverse-bias leakage current is reduced by three orders of magnitude on LEO GaN. Temperature-dependent measurements on the LEO diodes indicate that the remaining leakage current in these devices is associated with a deep trap level. © 1998 American Institute of Physics.

Notes: The metalorganic vapor phase epitaxy of coherent self-assembled InAs islands on InP(001) is demonstrated. Samples are characterized using transmission electron microscopy and photoluminescence (PL) spectroscopy at 77 K. The deposition of ∼2.4–4.8 monolayers (ML) of InAs at 500°C followed by a 30 s growth interruption results in the formation of coherent islands whose average diameter is 30–35 nm with a standard deviation of 8 nm and whose areal density is (3–4)×1010 cm−2. The PL emission is centered at 0.79 eV and has a full width at half maximum (FWHM) of 90 meV. When the nominal deposited thickness is increased to ∼9.6 ML, the average island diameter increases to ∼120 nm while the areal density decreases to ∼109 cm−2. The resulting PL is then centered at 0.83 eV with a FWHM of 130 meV and also displays a peak at 1.23 eV which is attributed to an InAs wetting layer ∼2 ML in thickness. © 1997 American Institute of Physics.

Notes: Solar-blind ultraviolet photodiodes with a band-edge wavelength of 285 nm were fabricated on laterally epitaxially overgrown GaN grown by metalorganic chemical vapor deposition. Current–voltage measurements of the diodes exhibited dark current densities as low as 10 nA/cm2 at −5 V. Spectral response measurements revealed peak responsivities of up to 0.05 A/W. Response times for these diodes were measured to be as low as 4.5 ns for 90%-to-10% fall time. For comparison, diodes were fabricated using the same p–i–n structure deposited on dislocated GaN. These diodes had dark current densities many orders of magnitude higher, as well as a less sharp cutoff, and a significant slow tail under impulse excitation. © 1999 American Institute of Physics.

Notes: The room-temperature thermal conductivity of lateral epitaxial overgrown metalorganic chemical vapor deposition GaN films is reported to be in excess of 155 W/m K. This compares with the reported value of 130 W/m K for bulk single crystals and a similar value (135 W/m K) for a thick (∼50 μm) GaN film grown, without a nucleation layer, on sapphire by hydride vapor phase epitaxy. The measurements were made by a third-harmonic electrical measurement. © 1999 American Institute of Physics.

Notes: A photoelectrochemical (PEC) wet-etching technique (backside-illuminated PEC) is described that utilizes the dopant or band-gap selectivity of PEC etching to fabricate deeply undercut structures. Lateral etch rates exceeding 5 μm/min have been observed, producing cantilevers in excess of 100μm in length. Dramatically different etch morphologies were noted between the frontside- and backside-illuminated etching, though dopant-dependent etch selectivities were maintained. © 2000 American Institute of Physics.