phys. stat. sol. (b) 216, 431 (1999)

Subject classification: 68.65.+g; 61.16.Ch; S7.14

GaN Quantum Structures with Fractional Dimension ±±From Quantum Well to Quantum Dot

S. Tanaka1� (a), I. Suemune (a), P. Ramvall (b), and Y. Aoyagi (b)

(a) Research Institute for Electronic Science, Hokkaido University, North 12 West 6,Kita-ku, Sapporo 060-0812, Japan

(b) The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako-shi,Saitama 351-0198, Japan

(Received July 4, 1999)

GaN quantum structures with various fractional dimensions were fabricated on AlxGa1ÿxN sur-faces. The AlxGa1ÿxN surface was treated with a Si antisurfactant prior to the GaN deposition.This treatment was found to be effective in modifying the structural dimensions of the thin GaNlayer. Without Si a GaN quantum well structure having a dimensionality of two was achieved instep flow growth mode. As the deposited amount of Si was increased, a morphological transitionfrom quantum well (2D) to quantum dot (0D-like) occurred. At some Si doses the resulting struc-tures possessed fractional dimensions. We observed that GaN quantum structures with variousfractional dimensions could be controllably fabricated solely by varying the total amount of thedeposited Si antisurfactant. A model concerning masking by Si±N bondings is introduced to ex-plain the morphological transitions.

1. Introduction

During recent years the progress in developing GaN-related optical devices, for exampleInGaN quantum well (QW) lasser diodes, has been significant. However, the lasing orluminescence mechanisms are still controversial in this material system mainly becauseof the presence of strong piezoelectric fields and compositional fluctuations, which affectthe optical transition energies. Exciton localization at the potential minima created by aphase separation in InGaN QWs is believed to have a significant impact in improvingthe performance of optical devices made from these materials. Quantum dot (QD) andother low-dimensional structures, in which the excitons are confined in a space of re-duced dimensionality, are considered to strongly modify the optical properties [1, 2]. It isknown that an integer-dimensional modeling does not perfectly describe the experimen-tal reality in such systems. Instead models utilizing fractional dimensions (FDs) havebeen found to be more suitable [3, 4]. The present work is a continuation of previousstudies of GaN QDs grown on AlGaN surfaces using an antisurfactant [5, 6]. Here wedescribe the formation mechanisms of GaN QDs and propose new quantum structureswith various FDs. These structures may constitute suitable systems to study in order toimprove the understanding of the relation between the luminescence energy and thedegree of exciton localization as previously investigated theoretically [4].

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1) Corresponding author; phone: +81-11-706-3372; Fax: +81-11-706-4973;e-mail: stanaka@es.hokudai.ac.jp

2. Experimental

The GaN structures were grown on AlxGa1ÿxN (x � 0:15) surfaces which were depos-ited on AlN/Si-face on-axis 6H-SiC(0001) substrates by a horizontal-type metal-organicchemical vapor deposition (MOCVD) system. Ammonia (NH3), trimethylaluminium(TMA), trimethylgallium (TMG), and tetraethylsilicon (TESi) were supplied with H2 asa carrier gas. N2 were independently supplied via a separate reactor tube and mixedjust before the substrate susceptor for the growth of AlN, GaN, and AlxGa1ÿxN solidsolutions. Typical gas flow rates were 2 standard (at 760 Torr) liters per minute (SLM),2 SLM, and 0.5 SLM for NH3, H2, and N2, respectively. The substrate temperature wasmeasured with a thermocouple located at the substrate susceptor. After depositing a�1.5 nm thick AIN buffer layer [7], an approximately 0.6 mm thick AlxGa1ÿxN layerwas grown. The atomically smooth surfaces (step and terrace feature) of the AlxGa1ÿxNlayers were confirmed by atomic force microscopy (AFM) [5]. To achieve FD structuresor QDs, the AlxGa1ÿxN surface was treated by tetraethylsilicon (Si(C2H5)4 : TESi) inthe absence of NH3 gas, followed by the deposition of �1 to 2 nm thick GaN. Theactual amount of Si adsorbed on the surface is difficult to estimate mainly due to thefact that the sticking coefficient of TESi on the AlxGa1ÿxN surface is not known. Theamounts given here are the feeding rates of TESi into the reactor. A rough estimationsuggests that 1 nmol of TESi corresponds to �0.001 ML. During growth the reactorpressure was maintained at 76 Torr, and the GaN growth and the TESi feeding tem-peratures were kept constant at 1080 �C.

3. Results

AFM graphs of the samples at various Si doses: 0, 3.2, 4.8, 8.1, and 32 nmol are shownin Fig. 1a to e. With no TESi treatment a clear step and terrace feature was observed.This indicates that the GaN growth mode on AlxGa1ÿxN surfaces under experimentalconditions used here is step-flow. Thus, GaN/AlxGa1ÿxN quantum well structures hav-ing an abrupt interface can be achieved. As the deposited amount of Si increased, amorphological transition from 2D to 0D occurred. A clear step flow feature (2D) asseen in Fig. 1a was transformed into a 0D-like structure in Fig. 1e via several FD struc-tures. The Figs. 1b to d demonstrate the FD structures. The dimensionality of eachstructure is difficult to determine, however, it is clear that each structure prossesses aFD between 0 and 2D. For example, Fig. 1b shows a quantum structure having a dimen-sion close to two because of the fluctuated step feature and the high density of nano-holes, which resulted from Si±N masking effects as will be discussed later. From thisresult we conclude that GaN quantum structures with various FDs can be controllablyachieved by varying the amount of Si deposited on the AlxGa1ÿxN surface beforegrowth of the final GaN layer.

4. Discussion

A possible mechanism to explain the morphological transition induced by Si will beconsidered in terms of a masking effect, which are schematically shown in Fig. 2a to c.Figs. 2a, b and c correspond to 0 nmol (2D), 2.2 to 4.8 nmol (quasi-2D), and 8.1 nmol

432 S. Tanaka et al.

(quasi-0D) cases, respectively. At the Si supply stage, Si±N bonds on top of the Ga(Al)dangling bonds of the AlxGa1ÿxN surface are formed. The model in which the Ga(Al)atoms at the surface are replaced with Si atoms is another possibility. The detailedmodel will be discussed in combination with a first principles calculation in the future.The nanoscopic changes in the surface structure and chemistry by the Si supply shouldenergetically and kinetically affects the GaN nucleation resulting in morphological tran-sition. Such an area caused by the nanoscopic modification of the surface may act as anano-scale mask where no GaN deposition is observed and may also create a kineticbarrier for adatom diffusion (see the masking territory in Figs. 2d and e). Here, we usethe former model, masking by the Si±N bondings, to explain the growth mechanisms.Fig. 2d shows a plan view of c) to illustrate the GaN nucleation sites at the crossingregion n, where GaN deposition more likely takes place due to a potential gradient(chemical potential) induced by Si±N bondings. The large amount of holes observed inFigs. 1b and c correspond to the area of masking. As the masking area is increased, 2Dand 0D-like structures appeared in the crossing region (denoted by ªnº in Fig. 2d)where the surface potential energy is relatively low due to the potential gradient in-duced by the Si±N bondings. Fig. 2e illustrates a possible surface potential energy dia-gram across the region along the line in Fig. 2d. DEchem and DEdiff denote the barrierheight induced by the Si±N bondings and the activation energy for diffusion of ada-toms, respectively.

GaN Quantum Structures with Fractional Dimension 433

Fig. 1. Effects of the Si dose on the GaN morphology. Note that the GaN structural dimension ischanged from 2D (part a) to 0D-like (part e)

5. Summary

GaN quantum structures with various fractional dimensions were demonstrated. Theuse of an antisurfactant on AlxGa1ÿxN surfaces was shown to be effective in modifyingstructural dimensions. A model concerning masking by Si atoms and its influence onthe surface potential energies was introduced to explain the morphological transitions.

Reference

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Fig. 2. Schematic figures of the model explaining the structural transitions observed by AFM.a), b) and c) correspond to 0 nmol (2D), 2.2 to 4.8 nmol (2D to 1D), and 8.1 nmol (1D to 0D) cases,respectively. Part d) shows a plan view of c) to illustrate the GaN nucleation sites at a crossingregion n, where GaN deposition more likely takes place due to a potential gradient induced bySi±N. Part e) illustrates a possible surface potential energy diagram across the region along theline A±B in d). DEchem and DEdiff denote the barrier height induced by Si atoms and the activa-tion energy for diffusion of adatoms in the crossing region, respectively

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