F. H. Long

Spatial characterization of doped SiC wafers by Raman spectroscopy

Raman spectroscopy has been used to investigate wafers of both 4H–SiC and 6H–SiC. The wafers studied were semi-insulating and n-type (nitrogen) doped with concentrations between 2.1×1018 and 1.2×1019 cm−3. Significant coupling of the A1 longitudinal optical (LO) phonon to the plasmon mode was observed. The position of this peak shows a direct correlation with the carrier concentration. Examination of the Raman spectra from different positions on the wafer yielded a rudimentary spatial map of the carrier concentration. These data are compared with a resistivity map of the wafer. These results suggest that Raman spectroscopy of the LO phonon–plasmon mode can be used as a noninvasive, in situ diagnostic for SiC wafer production and substrate evaluation.

Time-resolved photoluminescence measurements of InGaN light-emitting diodes

We have used time-resolved photoluminescence (PL) to examine light-emitting diodes made of InGaN/GaN multiple quantum wells (MQWs) before the final stages of processing. The time-resolved photoluminescence from a dim MQW was quenched by nonradiative recombination centers. The PL kinetics from a bright MQW were not single exponential but stretched exponential, with the stretch parameter β=0.59±0.05. The emission lifetime varied with energy, within error β was independent of the emission energy. the stretched exponential kinetics are consistent with significant disorder in the material. We attribute the disorder to spatial fluctuations of the local indium concentration.

Time-resolved photoluminescence measurements of quantum dots in InGaN multiple quantum wells and light-emitting diodes

We have used time-resolved photoluminescence to examine InGaN/GaN multiple quantum wells (MQWs) and light-emitting diodes (LEDs) before the final stages of processing at room temperature. The photoluminescence kinetics are well described by a stretched exponential exp[−(t/τ)β], indicating significant disorder in the material. We attribute the disorder to nanoscale quantum dots of high local indium concentration. For the three MQWs examined, the stretching parameter β and the stretched exponential lifetime τ were found to vary with emission energy. The stretching parameter β for the emission peak of the three MQWs was observed to increase from 0.75 to 0.85 with apparently increasing indium phase segregation. A higher degree of indium phase segregation is consistent with more isolated quantum dots inside the two-dimensional quantum well. The time-resolved photoluminescence from a LED wafer, before the final stages of processing, suggests the importance of quantum dots of high indium concentration on the LED...

Time-resolved spectroscopy of InxGa1−xN/GaN multiple quantum wells at room temperature

We have measured the time-resolved photoluminescence (PL) from a series of InxGa1−xN/GaN (x=0.22) multiple quantum well structures at room temperature. Lifetimes longer than 1 ns (1.87±0.02 ns) were measured at room temperature. The emission lifetime was found to lengthen with increasing excitation power, this is attributed to the saturation of recombination centers. The PL decay kinetics were found to be quite sensitive to the emission wavelength. The energy dependence of the emission lifetime is attributed to nanoscale fluctuations in the indium concentration.

RAMAN MICROSCOPY OF LATERAL EPITAXIAL OVERGROWTH OF GAN ON SAPPHIRE

We have used confocal Raman microscopy to investigate lateral epitaxially overgrown (LEO) GaN on sapphire substrates. The one-phonon Raman spectra are consistent with pyramidal growth of the GaN before coalescence has occurred. The position and asymmetric line shape of the A1 longitudinal optical (LO) phonon demonstrate that the LEO GaN is doped. The dopant is most likely Si from the SiN mask used to produce the LEO GaN. The carrier concentration is estimated to be 1×1017 cm−3. We have also used Raman microscopy to spatially resolve the yellow emission from different regions of the LEO GaN.

Resonance enhancement of electronic Raman scattering from nitrogen defect levels in silicon carbide

Electronic Raman scattering from nitrogen defect levels in SiC is seen to be significantly enhanced with excitation by red (633 nm, 1.98 eV) or near-IR (785 nm, 1.58 eV) laser light at room temperature. Four nitrogen peaks are observed in 6H–SiC (380, 430, 510, and 638 cm−1) and three peaks in 4H–SiC (about 400, 530, and 570 cm−1). The peaks in the 4H–SiC spectrum are seen to shift to lower frequency with increasing nominal doping concentration. Raman spectra taken at low temperature in 6H–SiC reveal differences between wafers and Lely grown platelets by the appearance of several additional peaks. The origin of the resonant enhancement is the near-IR absorption band associated with the green color characteristic of n-type SiC. These results demonstrate that the laser wavelength is a key parameter in the characterization of SiC by Raman scattering.

Disorder in InGaN light-emitting diodes

We have used time-resolved photoluminescence (PL), with 400 nm (3.1 eV) excitation, to examine InxGa1-xN/GaN light- emitting diodes (LEDs) before the final stages of processing at room temperature. We have found dramatic differences in the time-resolved kinetics between dim, bright and super bright LED devices. The lifetime of the emission for dim LEDs is quite short, 110 plus or minus 20 ps at photoluminescence (PL) maximum, and the kinetics are not dependent upon wavelengths. This lifetime is short compared to bright and super bright LEDs, which we have examined under similar conditions. The kinetics of bright and super bright LEDs are clearly wavelength dependent, highly non-exponential, and are on the nanosecond time scale (lifetimes are in order of 1 ns for bright and 10 ns for super bright LED at the PL max). The non- exponential PL kinetics can be described by a stretched exponential function, indicating significant disorder in the material. Stretched exponential lifetimes are consistent with a distribution of lifetimes. Typical values for (beta) , the stretching coefficient, are 0.45 - 0.6 for bright LEDs, at the PL maxima at room temperature. We attribute this disorder to indium alloy fluctuations. From analysis of the stretched exponential kinetics we estimate the potential fluctuations to be approximately 75 meV in the super bright LED. Assuming a tunneling based hopping mechanism, the average distance between indium quantum dots in the super bright LED is estimated to be 20 Angstrom.

Epitaxial film growth and characterization

Publisher Summary This chapter reviews the current status of epitaxial growth technology and the characterization of the deposited material. The production of cutting edge compound semiconductor devices requires the growth of high quality epitaxial layers. An epitaxial layer is one that takes the same structure as the substrate it is deposited on—that is, the same crystal symmetry and lattice constant. If the layer is the same material as the substrate, it is said to be homoepitaxial, and if the layer is a different material, it is heteroepitaxial. Other derivatives include strained-layer epitaxy, where elastically strained layers of different lattice constant also exist. The two principal techniques in widespread use today for the deposition of compound semiconductor materials are molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). MOCVD is a broader term that is applicable to the deposition of crystal, polycrystalline, and amorphous materials. Both MBE and MOCVD have produced a wide range of very high-purity semiconductor materials with excellent optical and electrical properties. Most research and development has centered on the growth of III–V semiconductor binary, ternary, and quaternary alloys with greatest emphasis on GaAs, (AlGa)As, and (GaIn)As.

Chapter 1 – Epitaxial Film Growth and Characterization

Publisher Summary Over the past few years, the market for compound semiconductor-based devices has continued to expand and mature, and much of the commercial promise for these materials is realized. Many devices have now reached the stage of significant manufacturing volumes, including light-emitting diodes, laser diodes, solar cells, and electronic devices, such as high electron mobility transistors and heterojunction bipolar transistors. All of these devices require the deposition of thin epitaxial layers, and these layers often have lower defect and impurity levels as compared to bulk materials. This chapter focuses on epitaxial film growth and its characterization. The deposition of these epitaxial layers could be performed through various deposition techniques such as vapor phase epitaxy, liquid phase epitaxy, molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition (MOCVD). Of these, MBE and MOCVD are dominant because they are capable of reproducibly generating the advanced device structures that require very thin layers and monolayer abrupt transitions in composition. The MBE has tended to dominate the growth of electronic devices where volumes are relatively low and a premium is placed on interface control. The MOCVD has tended to dominate the growth of optoelectronics devices where cost is more important and high capacity tools are required. High throughput production has raised a new challenge for whole wafer and nondestructive material characterization that is quite different from traditional single point and destructive measurements. In a production environment, the necessity of reliable and rapid turnaround completely nondestructive wafer mapping characterization, techniques have become apparent and are currently being developed.

Time-resolved photoluminescence measurements of InGaN light-emitting diodes, films, and multiple quantum wells

We have used time-resolved photoluminescence (PL) to examine light-emitting diodes made of InGaN/GaN multiple quantum wells (MQWs) before the final stages of processing. The time-resolved photoluminescence from a dim MQW was quenched by nonradiative recombination centers. The PL kinetics from a bright MQW were not single exponential but stretched exponential, with the stretch parameter (beta) equals 0.59 +/- 0.05. The emission lifetime varied with energy, within error (beta) was independent of the emission energy. The stretched exponential kinetics are consistent with significant disorder in the material. Related results for an InGaN film and InGaN/GaN MQWs are also reported. We attribute the disorder to fluctuations of the local indium concentration.