An overview is presented of band alignments for small-lattice parameter, refractory semiconductors. The band alignments are estimated empirically through the use of available Schottky barrier height data, and are compared to theoretically predicted values. Results for tetrahedrally bonded semiconductors with lattice constant values in the range from C through ZnSe are presented. Based on the estimated band alignments and the recently demonstrated p-type dopability of GaN, we propose three novel heterojunction schemes which seek to address inherent difficulties in doping or electrical contact to wide-gap semiconductors such as ZnO, ZnSe, and ZnS. © 1995 American Institute of Physics.This letter presents Schottky barrier based estimates of band alignments among tetrahedrally bonded semiconductors with lattice constant values in the range from C to ZnSe. This range includes the refractory semiconductors, e.g., GaN and related nitride compounds, of much current interest due to recent successes in short-wavelength light-emitting devices. 1 The band alignments are determined empirically from Au Schottky barrier height data in a manner similar to that found in Refs. 2 and 3. Basis for the connection between band alignments and Schottky barrier height data can be found in Ref. 4. For our study, we use primarily the limited Schottky barrier data currently available, which nonetheless appear sufficient to indicate some striking differences between small and large lattice-parameter semiconductors. These Schottkybased band edge energies are compared to the predictions of Harrison and Tersoff. 5 Based on our findings for valence band offsets of wide-gap semiconductors, three applications, each involving heterostructures of p-type GaN with wide-gap II-VI semiconductors, are proposed as a means of circumventing limitations in p-type dopability or electrical contact formation in the latter materials.The band alignments are summarized in Fig.
NBTI and PBTI are studied in IL/HK/MG gate stacks having EOT down to ∼ 6Å and fabricated using low T RTP based thermal IL and a novel IL/HK integration. At equivalent EOT, proposed stacks provide improved NBTI and similar PBTI when compared to conventional Chem-Ox IL based HKMG stacks. EOT scaling achieved by RTP thermal IL scaling shows lower rate of increase in NBTI and PBTI when compared to Chem-Ox IL scavenged stacks. Impact of Nitrogen and role of post HK nitridation are studied. Physical mechanism of improved BTI in proposed stacks is discussed in detail.
We have used x-ray photoelectron spectroscopy to measure the valence-band offsets for the lattice matched MgSe/Cd,,&!&.&e and MgTe/Cd,,ssZQ.,,Te heterojunctions grown by molecular beam epitaxy. By measuring core level to valence-band maxima and core level to core level binding energy separations, we obtain values of 0.5610.07 eV and 0.43IfrO.11 eV for the valence-band offsets of MgSe/Cdo,54Zno,,Se and MgTe/Cd,,ssZn O.lzTe, respectively. Both of these values deviate from the common anion rule, as may be expected given the unoccupied cation d orbitals in Mg. Application of our results to the design of current II-VI wide band-gap light emitters is discussed. For the ZnSe-based devices, knowledge of the valenceband offset (m,) arising from the addition of Mg in the Zn,-,M@,,Sel-, cladding layers is needed to design LDs with adequate carrier confinement, and to predict general trends in the dopability of the cladding layers. For the graded electron injector devices, AE, is needed to design for efficient electron injection. Currently, both types of devices are designed using band lineups based primarily on the common anion rule. Since the common anion rule' was originally observed not to apply to the one cation, Al, then studied from the third row of the periodic table, we should not expect it to necessarily apply to Mg, another third row cation. Similarly, Wei and Zunger" predict that the common anion rule should only apply when the d orbitals of the cations on both sides of a heterojunction, e.g., MgSe/Cd0.54Zn0,4$e, interact with their respective valence bands in a comparable manner. heterojunctions by x-ray photoelectron spectroscopy @PS)* Cdo.54Zno.4, Se and Cdos8Zno.1,Te were chosen to avoid difficulties associated with measuring band offsets in lattice mismatched systems, and to test the validity of the common anion rule for Mg-based compounds.The structures studied were grown in two Perk&Elmer 430P MBE systems, one devoted to III-V and the other to II-VI semiconductor growth. After GaSb buffer layers were grown on GaSb (100) substrates to provide a smooth growth surface, the samples were transferred via an ultrahigh vacuum (UHV) transfer tube to the II-VI growth chamber. Thick, relaxed layers of Cdo.54Zn,,46Se (Cdo~88Zno~1,Te), followed by the band offset measurement structures, were then grown at a substrate temperature of 270 "C (300 "C). The ternary compositions were calibrated using XPS and x-ray diffraction to keep the lattice mismatch to less than 0.5%. The reflection high-energy electron diffraction patterns showed no indications of deviation from the cubic zincblende structure. Following the II-VI growth, the samples were transferred via an UHV transfer tube to a Perkin-Elmer model 5500 analysis system with a monochromatic Al Ka x-ray source for the ?LpS measurements. The base pressure in the XPS chamber was typically -3X1O-1o Torr.To determine bE, using XPS, we used the following relation:
We propose a new device structure for obtaining visible light emission from wide band gap semiconductors. This heterojunction structure avoids ohmic contacting problems by using only the doping types which tend to occur naturally in II-VI semiconductors, while using a novel injection scheme to obtain efficient minority carrier injection into the wider band gap semiconductor. To verify this proposal we have fabricated green light emitting structures using n-CdSe and p-ZnTe regions separated by a graded Mg,Cdt -$e injection region. Room temperature electroluminescence spectra from these devices demonstrate the effectiveness of the injection scheme, while the current-voltage characteristics show the merits of avoiding difficult ohmic contacts. We further show how the structure can be extended to blue wavelengths and beyond by opening up the band gap of the ZnTe recombination region with a Mg,,Zn,-,,Te alloy.
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