L. Bergman

The origin of the broadband luminescence and the effect of nitrogen doping on the optical properties of diamond films

Raman and various photoluminescence (PL) techniques were employed to investigate the role of nitrogen doping on the optical spectra of chemical‐vapor‐deposited (CVD) diamond films and to determine the origin of the characteristic broadband luminescence which is observed from approximately 1.5 to 2.5 eV and centered at ∼2 eV. The PL transitions attributed to the zero‐phonon lines (ZPL) of nitrogen centers are observed at 1.945 and 2.154 eV. A new possible nitrogen center at 1.967 eV is also observed as well as the band A luminescence centered at ∼2.46 eV. The experimental results preclude the possibility of the broadband PL being due to electron‐lattice interaction of the nitrogen ZPL centers. We establish the presence of an in‐gap state distribution in CVD diamond films attributed to the sp2 disordered phase and show that its optical transitions are the likely cause of the broadband luminescence. A model of the in‐gap state distribution is presented which is similar to models previously developed for amor...

Raman and photoluminescence analysis of stress state and impurity distribution in diamond thin films

Photoluminescence (PL) and Raman spectroscopy were employed to investigate the nature and sources of stress and the type and distribution of impurities and defects in thin diamond films grown on silicon substrates. The types of impurities and defects which were detected in the diamond films are the nitrogen, silicon, and the sp2‐type bonding of the graphitic phase. Our Raman analyses indicate that the diamond films exhibit a net compressive stress. After compensating for the thermal interfacial stress and for the stress due to grain boundaries it was found that the residual internal stress is compressive in nature. From Raman line‐shape analysis it was determined that the internal stress is due to the various impurities and defects present in the film. Moreover, the stress magnitude exhibits a strong correlation with the graphitic phase implying that the sp2 bonding produces a dominant compressive stress field. The PL analytical line‐shape investigation of the nitrogen band at 2.154 eV indicates that the ...

Sublimation growth and characterization of bulk aluminum nitride single crystals

Single crystalline platelets of aluminum nitride (AlN) ⩽ 1 mm thick have been grown within the temperature range of 1950–2250°C on 10 × 10 mm2 α(6H)-silicon carbide (SiC) substrates via sublimation-recondensation in a resistively heated graphite furnace. The source material was sintered AlN. A maximum growth rate of 500 μm/h was achieved at 2150°C and a source-to-seed separation of 4 mm. Growth rates below 2000°C were approximately one order of magnitude lower. Crystals grown at high temperatures ranged in color from blue to green due to the incorporation of Si and C from the SiC substrates; those grown at lower temperatures were colorless and transparent. Secondary-ion mass spectroscopy (SIMS) results showed almost a two order of magnitude decrease in the concentrations of these two impurities in the transparent crystals. Plan view transmission electron microscopy (TEM) of these crystals showed no line or planar defects. Raman spectroscopy and X-ray diffraction (XRD) studies indicated a strain free material.

Microphotoluminescence and Raman scattering study of defect formation in diamond films

Photoluminescence and Raman spectroscopy are employed to explore the time evolution of defect formation in chemical vapor deposition diamond films for stages of growth spanning nucleation to continuous film formation. Our research is concentrated on three types of defects which give rise to the 1.68 eV optical band, the sp2 phase which centers at 1500 cm−1, and the broadband luminescence at 565–800 nm. The investigation of these types of defects suggests the following conclusions. Si atoms are most likely responsible for the creation of the 1.68 eV optical centers which takes place at the initial stages of growth. Plasma interactions with the Si substrate contribute to the 1.68 eV defect formation. The broad luminescence and sp2 bonding defects were not present in the isolated nuclei but were significantly present when a continuous film was formed. Two rates of diamond growth were obtained and the changes of the rates were attributed to the lowering degree of freedom available for the growth of the nuclei...

Raman analysis of the E1 and A1 quasi-longitudinal optical and quasi-transverse optical modes in wurtzite AlN

This article presents a study of the quasi-longitudinal optical and quasi-transverse optical modes in wurtzite AlN which originate from the interaction of phonons belonging to the A1 and E1 symmetry groups. In order to analyze the allowed quasi as well as pure Raman modes, the modes were observed in a rotating crystallographic coordinate system, and the Raman tensors of the wurtzite crystal structure were calculated as a function of the crystallographic rotation. The frequencies of the quasimodes of wurtzite AlN were also analyzed in terms of the interaction of the polar phonons with the long range electrostatic field model. The experimental values of the Raman frequencies of the quasiphonons concur with these expected from the model, implying that the long range electrostatic field dominates the short range forces for polar phonons in AlN.

Raman analysis of the configurational disorder in AlxGa1−xN films

Raman analysis of the E2 mode of AlxGa1−xN in the composition range 0⩽x⩽1 is presented. The line shape was observed to exhibit a significant asymmetry and broadening toward the high energy range. The spatial correlation model is discussed, and is shown to account for the line shape. The model calculations also indicate the lack of a long-range order in the chemical vapor deposition alloys. These results were confirmed by x-ray scattering: the relative intensity of the superlattice line was found to be negligible. The line broadening of the E2 mode was found to exhibit a maximum at a composition x≅0.5 indicative of a random disordered alloy system.

Effects of boron doping on the surface morphology and structural imperfections of diamond films

This paper reports the surface morphology and structural imperfection of boron-doped diamond films prepared by microwave plasma enhanced chemical vapor deposition. It was found that boron dopants improved the structural quality of diamond films. The surface morphology consisted mainly of the {111} facets. A significant enhancement of nucleation density and consequent decrease of grain size was observed with the addition of diborane in the gas phase. Raman spectroscopy indicated that, with the introduction of boron dopants, the integrated intensity of the diamond peak at 1332 cm−1 increased relative to the intensity of the non-diamond peak at about 1500 cm−1, and the full-width at half maximum of the 1332 cm−1 peak decreased. In addition, the 1.681 eV (738 nm) photoluminescence peak related to point defects was effectively reduced, or even eliminated by the boron dopants. Finally, transmission electron microscopy studies found that the densities of planar defects (mainly stacking faults and microtwins) also decreased with the boron addition.

Ultraviolet Raman study of A1(LO) and E2 phonons in InxGa1−xN alloys

We report on ultraviolet Raman spectroscopy of InxGa1−xN thin films grown on sapphire by metal-organic chemical vapor deposition. The A1(LO) and E2 phonon mode behavior was investigated over a large compositional range (0<x<0.50). Compelling evidence is presented for one-mode behavior for the A1(LO) phonon, and data suggestive of two-mode behavior are presented for the E2 phonon.

Observation of surface modification and nucleation during deposition of diamond on silicon by scanning tunneling microscopy

The surface topography of silicon substrates after the initial stages of diamond growth, by microwave plasma enhanced chemical vapor deposition, has been observed by scanning tunneling microscopy. The initial surfaces were polished with diamond paste before deposition to enhance nucleation, and the scratches were examined. After one half hour growth, the surface showed additional topography over all regions, and widely spaced faceted structures were detected which were attributed to diamond nuclei. The surface between nuclei showed increased roughness with increased deposition time. The faceted nuclei were found along the scratches. The nuclei showed facets which were smooth to within 5 A. Fingerlike ridge structures were found extending from and in‐between some of the nuclei. These structures indicate a mechanism of the lateral diamond growth. The electronic properties of the surface were probed by local I‐V measurements, and characteristics attributable to SiC were observed.

Photoluminescence and recombination mechanisms in GaN/Al0.2Ga0.8N superlattice

A detailed study of photoluminescence (PL) of GaN(1 nm)/Al0.2Ga0.8N(3.3 nm) twenty periods superlattice grown via metal-organic chemical vapor deposition is presented. The dependence of the PL emission energy, linewidth, and intensity on temperature, in the low temperature regime, is consistent with recombination mechanisms involving bandtail states attributed to a small degree of interfacial disorder. The activation energy of the nonradiative centers in our superlattice agrees well with the value we derive for the width of the tail-state distribution. Moreover, we find that the average phonon energy of the phonons that control the interband PL energy at high temperatures is larger for the superlattice than for a high-quality GaN film. This observation is consistent with model calculations predicting the phonon mode properties of GaN–AlN-based wurtzite heterostructures.