A-oriented InP Substrate Research Articles

Effects of Heterointerface Flatness on Device Performance of InP-Based High Electron Mobility Transistor

We achieved a current gain cutoff frequency ( fT) of 310 GHz at a cryogenic temperature (16 K) in a 195-nm-long gate In0.75Ga0.25As/In0.52Al0.48As high-electron-mobility transistor (HEMT) fabricated on a (411)A-oriented InP substrate. This is 27% higher than that at room temperature (245 GHz). For a corresponding HEMT fabricated on a (100) InP substrate, fT of 268 GHz was also obtained at 16 K, which is 15% higher than that obtained at 300 K (233 GHz). For both (411)A and (100) HEMTs, this significant enhancement in low-temperature fTs was caused by the increased average electron velocity under the gate (vave), which was mainly due to the suppression of phonon scattering. Furthermore, fT as high as 310 GHz was attributed to vave of up to 4.9×107 cm/s at 16 K for the (411)A HEMTs. This is higher than that of the (100) HEMTs (4.5×107 cm/s) and is due to the reduced interface roughness scattering of electrons caused by (411)A super-flat InGaAs/InAlAs interfaces (effectively atomically flat heterointerfaces over a wafer-size area).

Thermal stability of Ti/Pt/Au ohmic contacts for cryogenically cooled InP-based HEMTs on (4 1 1)A-oriented substrates by MBE

We investigated the contact resistance of a non-alloyed Ti/Pt/Au ohmic electrode to obtain thermally stable source–drain resistance of cryogenically cooled In 0.75 Ga 0.25 As / In 0.52 Al 0.48 As high electron mobility transistors (HEMTs) fabricated on (4 1 1)A-oriented InP substrates by molecular beam epitaxy. A contact resistance of 0.26 Ω mm , which strongly depended on InAlAs spacer and barrier layers, was achieved at 16 K (25% below the value at 300 K) for the non-alloyed Ti/Pt/Au ohmic electrode formed on the In 0.75 Ga 0.25 As / In 0.52 Al 0.48 As HEMT structure with 3-nm-thick InAlAs spacer and 10-nm-thick InAlAs barrier layers. For a 195-nm-gate HEMT, we achieved a maximum transconductance ( g m ) of 2.25 S / mm at 16 K (26% above the value at 300 K), which, to our knowledge, is one of the highest values for HEMTs ever reported. This extremely high g m was attributed to not only 33% lower source–drain resistance ( 0.18 Ω mm at 16 K) because of thermally stable and low contact resistance but also 19% lower transit time under the gate (0.39 ps at 16 K) because of phonon scattering suppression compared with the 300 K values.

Dramatic dependence of the Fermi level pinning strength on crystal orientation at clean surfaces of n-type In 0.53Ga 0.47As grown by molecular beam epitaxy

The electronic density of states at clean surfaces of n-type In 0.53Ga 0.47As, grown by molecular beam epitaxy on lattice-matched (0 0 1)- and (1 1 1)A-oriented InP substrates, was measured by low-temperature scanning tunneling spectroscopy under ultra-high vacuum. It was found that the surface Fermi level (FL) pinning strength dramatically depends on crystal orientation. The FL at the (0 0 1)-(2×4) surface is pinned near midgap, independently of the dopant concentration in the bulk. In contrast, the FL at the (1 1 1)A-(2×2) surface lies in the conduction band, close to the bulk FL, and increases with dopant concentration.

Velocity Enhancement in Cryogenically Cooled InP-Based HEMTs on (411)A-Oriented Substrates

An extremely high maximum transconductance gmmax of 2.25 S/mm and a cutoff frequency fT of 310 GHz was achieved at a cryogenic temperature (16 K) in a 195-nm-gate In0.75Ga 0.25As/In0.52Al0.48As high electron mobility transistor (HEMT) fabricated on a (411)A-oriented InP substrate by molecular beam epitaxy, compared with room temperature values (gmmax=1.78 S/mm and fT=245 GHz). These significantly enhanced gmmax and fT values are attributed to a high electron velocity of up to 4.9times107 cm/s due to suppressing phonon scattering in the In0.75Ga0.25As/In0.52Al0.48 As HEMT with (411) A super-flat InGaAs/InAlAs interfaces (effectively atomically flat heterointerfaces over a wafer-size area)

1.3 μm range effectively cylindrical In 0.53Ga 0.47As/In 0.52Al 0.48As quantum wires grown on (2 2 1)A InP substrates by molecular beam epitaxy

Vertically coupled quantum wires (QWRs) have been made by alternately stacking nominally 3.6 nm thick In 0.53Ga 0.47As self-organized QWR layers and 1 nm thick In 0.52Al 0.48As barrier layers on (2 2 1)A-oriented InP substrates by molecular beam epitaxy. The surface of In 0.53Ga 0.47As QWR layers was corrugated at an amplitude of 1.1 nm and period of 27 nm, and lateral confinement potential is induced by their thickness modulation. The wavelength of photoluminescence (PL) from the stacked QWRs at 15 K becomes longer from 1220 to 1327 nm with increasing total number of stacked QWR layers, N SL, from 1 to 9, while PL full-width at half-maximum is reduced from 22 to 8.6 meV. The PL intensity with the polarization parallel to the wire direction, I ∥, is 1.30 times larger than that with the normal polarization, I ⊥, when N SL = 1 . The PL intensity ratio, I ∥/ I ⊥, reaches as large as 4 when N SL = 9 , indicating successful control of relative strength between vertical confinement and lateral confinement of carriers. The value of I ∥/ I ⊥ obtained for the stacked QWRs with N SL = 9 is the same value as cylindrical QWRs have. The results indicate that effectively cylindrical QWRs with the best uniformity and 1.3 μm range emission were realized by stacking of self-organized QWR layers.

High transconductance of 2.25 S/mm observed at 16 K for 195-nm-gate In/sub 0.75/Ga/sub 0.25/As/In/sub 0.52/Al/sub 0.48/As HEMT fabricated on [411]A-oriented InP substrate

We achieved a maximum transconductance (g/sub m/) of 2.25 S/mm at 16 K for a 195-nm-gate In/sub 0.75/Ga/sub 0.25/As/In/sub 0.52/Al/sub 0.48/As pseudomorphic high-electron mobility transistor (PHEMT) fabricated on a [411]A-oriented InP substrate, which is the highest value ever reported for HEMTs. This PHEMT also showed a much enhanced cutoff frequency (f/sub T/) of 310 GHz at 16 K, compared with its room temperature value (245 GHz). The significantly enhanced g/sub m/ and f/sub T/ at 16 K can be attributed to the higher saturation velocity in the region "under the gate," which is caused not only by suppressing the phonon scattering, but also by suppressing the interface roughness scattering due to the "(411)A super-flat InGaAs/InAlAs interfaces" (effectively atomically flat heterointerfaces over a wafer-size area).

Mobility enhancement by reduced remote impurity scattering in a pseudomorphic In0.7Ga0.3As/In0.52Al0.48As quantum well high electron mobility transistor structure with (411)A super-flat interfaces grown by molecular-beam epitaxy

We have carried out a Shubnikov–de Haas (SdH) measurement at 4 K and investigated the electronic properties and scattering mechanisms in a pseudomorphic In0.7Ga0.3As/In0.52Al0.48As quantum well high electron mobility transistor (QW-HEMT) structure with a thin spacer thickness of 3 nm grown on a (411)A-oriented InP substrate by molecular-beam epitaxy (MBE). Electrons occupied the zeroth and first subbands in the 12-nm-thick In0.7Ga0.3As channel layer at two-dimensional electron gas (2DEG) densities of 3.10×1012 and 0.99×1012 cm−2, respectively. 2DEG mobilities of the (411)A sample for the zeroth and first subbands were μ0=52 000 and μ1=66 000 cm2/V s, which were much higher than those of the (100) QW-HEMT structure (μ0=22 000 and μ1=26 000 cm2/V s). The result indicates that the electron mobility of the (411)A sample is enhanced by reduction of remote impurity scattering because the spacer thickness (Lsp=3 nm) and distribution of sheet doped impurities are laterally uniform in the (411)A In0.7Ga0.3As/In0.52Al0.48As QW-HEMT structure.

High-quality InAlAs layers grown on (411)A-oriented InP substrates by molecular beam epitaxy

High-quality InAlAs layers lattice matched to InP were successfully grown on (411)A-oriented InP substrates by molecular beam epitaxy (MBE). High-resolution x-ray diffraction and photoluminescence (PL) measurements of InAlAs layers grown on (411)A InP substrates revealed that crystalline quality of the (411)A InAlAs layer strongly depends on the substrate temperature (Ts), and much improved crystalline quality of (411)A InAlAs layer was achieved at a high Ts (570 °C). The linewidth of the PL (12 K) peak from the best (411)A InAlAs layer is 10.7 meV which is 16%–29% smaller than those (12.8–15 meV) of InAlAs layers grown on conventional (100) InP substrates by MBE.

High-Quality InGaAs Layers Grown on (411)A-Oriented InP Substrates by Molecular Beam Epitaxy

High-quality InxGa1-xAs layers (0.505 ≤x ≤0.545) were grown on (411)A-oriented InP substrates by molecular beam epitaxy (MBE). High-resolution X-ray diffraction (HRXRD) measurements of the (411)A InxGa1-xAs layers showed no relaxation of lattice mismatch, and residual strain components observed in the (411)A InGaAs layers were in good agreement with calculated results based on the constrained pseudomorphic layer model on high index substrates proposed by Yang et al. [Appl. Phys. Lett. 65 (1994) 2789]. Photoluminescence (PL) linewidth of the (411)A InGaAs layer with In content (x) of 0.505 was 1.9 meV at 12 K, which is less than half that (4.0 meV) of an InGaAs layer simultaneously grown on a (100) InP substrate, and almost the same as the optimum value (1.2–3.0 meV) reported for InGaAs layers grown on (100) InP substrates. These results indicate that high-quality InGaAs layers can be easily obtained by MBE growth on (411)A InP substrates.

Liquid-phase-epitaxial growth of Inp/InGaAsP/InGaAs buried-structure avalanche photodiodes

An InP/InGaAsP/InGaAs avalanche photodiode with an effective guard-ring structure has been successfully fabricated. The diode has a planar structure with an n-InP layer buried by n−-InP in the multiplication region The structure has been grown on a (111)A-oriented InP substrate by two-step growth of liquid-phase epitaxy. Prior to the second growth of n−-InP a meltback technique was used to reduce dark current.