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Probing for answers in the characterisation of advanced III–V device structures
Probing for Answers in the Characterisation of Advanced I Il-V Device Structures Derek K. Skinner and Alan V. Hetherington The successful production of advanced III-V devices such as the MQW laser relies heavily on the application of a wide range of analytical techniques for general characterisation and failure analysis. Single devices very often contain binary, ternary and quaternary III-V compounds in very thin layers of varying composition and intricate geometry. This article describes some of the methods that have been developed at GMMT Caswell by the materials analysis group to prepare these difficult materials for analysis. T
he last decade has seen many published articles relating the respective merits of Si and IIIV based semiconductor technology. The October 1989 issue of III-Vs Review, for example, carried a discourse which made a direct comparison of the two technologies from the point of view of the IC designer. This article is concerned with the analyst's point of view in dealing with complex II1-V compounds which, compared with Si, have special requirements in handling and preparation procedures. In addition, the complexity of many of the manufactured devices has dictated that only by adopting a multiprobe approach to characterisation can the necessary depth of information be obtained.
Material Differences The strength, predictable handling characteristic, simple chemistry, uniform sputtering habit, and general ease of handling are all qualities of Si the semiconductor industry has taken for granted over the years. It is
DFB grating for single
,ayer
0hml¢ contact metal
Figure 1. Schematic o['laser device.
perhaps hardly necessary to describe just how dissimilar IlI-V materials are to anyone involved in the industry. However, these differences present a challenge to the analyst due to the wide range of binary, ternary and quaternary compounds now in use, each with its own unique set of
Page 41
Vol 6 No 4
physical and chemical characteristics. Such diversity of material requires considerable experience and expertise in the preparation of samples for analysis. Sputtering artifacts, for example, vary enormously across the range of III-V compounds and where sputter-
ing is involved in sample preparation or analysis it is essential that these effects have been studied in previous experiments.
The Major Techniques The five m a j o r p r o b e techniques available at Caswell are as follows: • Scanning electron microscopy (SEM) • Transmission electron microscopy (TEM) • Auger electron spectroscopy (AES) • Electron m i c r o p r o b e (EDX and WDX) • X-ray analysis. The capabilities of these techniques are varied and complementary, each with its own particular strength. A coordinated a p p r o a c h is often employed, with several techniques being applied to the job in hand. In S E M , a focussed b e a m o f electrons is rastered across the sample surface resulting in the emission of secondary electrons. The yield of such electrons depends on the topographical features of the sample. By displaying the v a r i a t i o n o f s e c o n d a r y electron yield as a function of the electron beam position, a magnified image of the sample can be produced. With modern field emission instruments a resolution of" 2nm can be achieved. The strength of TEM lies in its ability to see inside a material. It can be used to assess the crystalline quality;, identify the phases present from diffraction patterns, or provide high resolution thickness measurements. To provide information on the internal structure of the material, a sample is thinned sufficiently so that a beam of electrons can be made to pass through it. The transmitted electrons are recorded on film to reveal the internal structure of the sample with a resolution of 0.5 nm. In AES a solid surface is subjected to an electron beam resulting in the ionisation of surface atoms. One process by which these atoms can return to their ground state is by the emission of so called Auger electrons. The energy of the Auger electron is characteristic of the emitting atom, and this technique enables the constituent elements forming the top 1 to 4 atomic layers of a sample to be identified. Extending analysis into the bulk of the sample is accomplished by a process of simultaneous sputter
Buried Structure
oooA Graded Layer
i/ P Atom %
lib
:.....
> Distance
~
In 446 As 7.7 P 42.3
C
zoA Gal Energy
= Distance
Fig zre 2a. Schematic ()/an ion beam bevelled section, showing the magn(/~'ing
P a g e 42
Vol 6 No 4
these X-rays are characteristic of the elements present in the sample and are determined by a spectrometer (wavelength dispersive) or a crystal detector (energy dispersive) depending on the nature of the sample. By making use of secondary electrons also generated in the process, a highly magnified image of the sample is produced. This enables excellent positional accuracy of the electron beam to be achieved. The following examples are included to provide a sample of the preparation methods used in dealing with III-V materials:
Electron Beam
BB
/~
.GaAssubstrata
Cleaved "~'~"~m-.,~ specimen ~ : 3 ~ i r ' / k
.---"S~ 'IR~--"~ _J'J.~" \ ~ I L " ~
Quanu tmwel \ Top
)Surface
F(gure 3. Schematic qfcleaved-ed~:e TEM method.
etching and analysis, producing the so called "Auger depth profile". In X-ray diffraction the incident Xrays are diffracted by the regular arrangement of atoms in a crystal. The intensity and angle at which diffraction occurs provides information about interatomic distances in the sample and enables us to define a range of compositional and structural information while X-ray topography can image the strain associated with defects in single crystals. Electron probe microanalysis utilises the X-rays generated in the top 13 microns of the sample during bombardment with a finely focussed beam of electrons. The energies of
Bevel sectioning In Fig. 1 a schematic diagram is shown of a typical advanced III-V laser structure. Characterisation of such a device would start in the early stages of production with qualification and quantification of the various layers and would include most of the techniques described in this article. Routine analytical requirements for quality control would typically require information on layer thickness and composition, lattice mismatch, homogeneity, crystalline quality and the integrity of heterointerfaces etc. AES can in addition be used to
produce in-depth compositional profiles. Of particular interest in this laser structure is the composition graded layer, which provides a graded refractive index of the confinement layers. The aim being to produce GaInAsP material that is linearly graded between the solid phase composition of InP and Ga0.181n0.82As0.40P0.60 whilst maintaining lattice matching conditions to the InP substrate. The method we have developed of quantifying the linearity of the grade is that outlined in Fig. 2, the so called "ion beam bevel section". This method uses a finely focussed ion beam electronically rastered in such a way as to sputter a very shallow linear bevel through the top layers to expose the region of interest. The bevel produces a magnified projection of the buried structure with layer magnifications of around 20,000 times. This method is used in combination with AES and enables the precise measurement of the composition of the buried layers. Compositional line scans across the region of interest are generated by scanning an electron beam across the bevel with the Auger spectrometer locked on to the element of interest. Fig. 2b shows the line scan for p h o s p h o r u s r e c o r d e d f r o m the
ANALYSIS of AI in AIGaAs CLEAVED EDGE TEM Dark Field g 200
AUGER PROFILE
image
AIGaAs InGaAs
E U:z E
10L
o-.e. E O
GaAs
10rim
.-
I
[
I
10
20 Depth in Layer nm
30
Figure 4a. Cleaved-edge TEM image of composition variation in AIGaAs layer. Figure 4b. Auger pro/ile showing extent of A I instability in A IGaAs layer
Page 43 I
Vol 6 No 4
graded region. Quantification of the grade is accomplished by taking selected points between its limits for a full analysis of all the elements. Fig. 2c is a band energy diagram of the planned structure with the Auger line scan excursion of Fig. 2b outlined. The a d v a n t a g e s of the bevelling approach over conventional Auger profiling are: • A high data density can be accumulated from the regions of specific interest, • Any number of elements can be profiled under optimised conditions, • Unsuspected contaminants can be found and identified, • The sample remains available for re-evaluation using the same bevel, • There is a faster sample turn-round than for conventional profiling.
Cleaved-edge TEM A fast accurate method of examining the epilayers in a III-V structure is the cleaved-edge TEM method. Fig. 3 shows how the method works. A small piece of wafer is polished from the back to a thicknes's of 100 p.m, and cleaved along < 1 1 0 > directions to form a small specimen with fresh, clean edges. The specimen is mounted on a copper grid as shown in the diagram, and this allows the electron beam of the TEM to penetrate the specimen near the edge where the thickness is less than 0.25 p.m. The image is recorded on film, and typical information which can be obtained from this technique includes: 1. The thickness of the epi-layers 2. The uniformity of the compositions 3. Whether the interfaces are fiat or uneven F'ig. 4a shows a TEM micrograph of an AIGaAs quantum well struclure. The thickness of the quantum well is easily measured to an accuracy of a few percent by this technique. However in this case, banding can be seen in the well structure. This is due to a regular variation of the composition through the well. TEM can give feedback within a few hours, and in this example uncovered a serious problem in the growth process. Fig. 4b is an Auger profile of the first few layers and shows the range of the AI instability in atomic %. The aluminium supply had developed an
Figure 5, Cros,s-seetion TEM o/quaternary &ser strueture.
instability, and after a short discussion with the growers the problem was identified, a change was made and the banding disappeared. The cleaved-edge TEM technique is a fast way of obtaining basic information about a III-V structure. No chemical staining is necessary and the sensitivity to composition and accuracy of measurement now make it the preferred technique over optical and SEM microscopies. When more detailed information is required, planview and cross-section TEM can be used.
Plan-view TEM In plan-view TEM a 3 mm disc of material is polished from the back to a thickness of 100 p.m, then chemically etched with a solution o f chlorine in methanol to perforation. The relatively large area near the hole can be examined for features such as dislocations and impurity phases. By tilting the specimen in the T E M and carefully selecting the diffraction condition used to form the image, dislocations can be analysed to determine their type and glide plane and from this information their source can often be determined.
Cross-section TEM Another method for examining the structures in III-V semiconductors is cross-section TEM. The unique feature of the procedure is the ability to look parallel to the growth interfaces Page 44
Vol 6 No 4
over a large distance with a resolution of 0.5 nm. In this case two pieces of material are glued face to face and polished perpendicular to the surface with a succession of SiC paper, alumina paste and diamond paste. This results in a 30 ~tm thick specimen highly polished on both sides with the epi interfaces perpendicular to the plane of the specimen. This is then ion milled to perforation, leaving thin area next to the hole suitable for TEM. Because of the instability of I I I - V c o m p o u n d s c o m p a r e d with silicon the ion milling is carried out at liquid nitrogen temperature to prevent dissociation of the material. Fig. 5 is a cross-section transmission electron micrograph from the quantum well structure whose compositional profile, analysed by Auger spectroscopy, was shown in Fig. 2. F r o m this cross section a b r o a d spectrum of assessment is possible. Structural quality such as the absence of dislocations, accurate measurement of layer thicknesses and the abruptness of the heterointerfaces in the multiple quantum well region can all be determined using this method. Typical problems that can be discovered by this method include: 1. Diffuse interfaces 2. Uneven layers - eg. if a layer has grown in a 3-D fashion 3. Dislocations generated within the structure 4. Compositional variations The examples presented in this article represent only a flavour of the wide ranging analysis required to fully characterise the complex layered structures on which modern III-V devices are based. It is essential that a range of complementary methods be available for such analysis. By the combined efforts of techniques such as multiple crystal x-ray diffraction and cross-section TEM for example, it is possible to measure the strain in these multi-layered structures and to show the position of misfit and threading dislocations which form to relieve the stress. Only with a full understanding of the elemental and physical composition can sufficiently accurate specifications for optimum device performance be determined. Contact: Peter Augustus. G M M T Ltd,, Caswell, Towcester, Northants, NN12 8EQ UK. Tel,(/a.w [44] ¢0)327 356362 356775 .
he last decade has seen many published articles relating the respective merits of Si and IIIV based semiconductor technology. The October 1989 issue of III-Vs Review, for example, carried a discourse which made a direct comparison of the two technologies from the point of view of the IC designer. This article is concerned with the analyst's point of view in dealing with complex II1-V compounds which, compared with Si, have special requirements in handling and preparation procedures. In addition, the complexity of many of the manufactured devices has dictated that only by adopting a multiprobe approach to characterisation can the necessary depth of information be obtained.
Material Differences The strength, predictable handling characteristic, simple chemistry, uniform sputtering habit, and general ease of handling are all qualities of Si the semiconductor industry has taken for granted over the years. It is
DFB grating for single
,ayer
0hml¢ contact metal
Figure 1. Schematic o['laser device.
perhaps hardly necessary to describe just how dissimilar IlI-V materials are to anyone involved in the industry. However, these differences present a challenge to the analyst due to the wide range of binary, ternary and quaternary compounds now in use, each with its own unique set of
Page 41
Vol 6 No 4
physical and chemical characteristics. Such diversity of material requires considerable experience and expertise in the preparation of samples for analysis. Sputtering artifacts, for example, vary enormously across the range of III-V compounds and where sputter-
ing is involved in sample preparation or analysis it is essential that these effects have been studied in previous experiments.
The Major Techniques The five m a j o r p r o b e techniques available at Caswell are as follows: • Scanning electron microscopy (SEM) • Transmission electron microscopy (TEM) • Auger electron spectroscopy (AES) • Electron m i c r o p r o b e (EDX and WDX) • X-ray analysis. The capabilities of these techniques are varied and complementary, each with its own particular strength. A coordinated a p p r o a c h is often employed, with several techniques being applied to the job in hand. In S E M , a focussed b e a m o f electrons is rastered across the sample surface resulting in the emission of secondary electrons. The yield of such electrons depends on the topographical features of the sample. By displaying the v a r i a t i o n o f s e c o n d a r y electron yield as a function of the electron beam position, a magnified image of the sample can be produced. With modern field emission instruments a resolution of" 2nm can be achieved. The strength of TEM lies in its ability to see inside a material. It can be used to assess the crystalline quality;, identify the phases present from diffraction patterns, or provide high resolution thickness measurements. To provide information on the internal structure of the material, a sample is thinned sufficiently so that a beam of electrons can be made to pass through it. The transmitted electrons are recorded on film to reveal the internal structure of the sample with a resolution of 0.5 nm. In AES a solid surface is subjected to an electron beam resulting in the ionisation of surface atoms. One process by which these atoms can return to their ground state is by the emission of so called Auger electrons. The energy of the Auger electron is characteristic of the emitting atom, and this technique enables the constituent elements forming the top 1 to 4 atomic layers of a sample to be identified. Extending analysis into the bulk of the sample is accomplished by a process of simultaneous sputter
Buried Structure
oooA Graded Layer
i/ P Atom %
lib
:.....
> Distance
~
In 446 As 7.7 P 42.3
C
zoA Gal Energy
= Distance
Fig zre 2a. Schematic ()/an ion beam bevelled section, showing the magn(/~'ing
P a g e 42
Vol 6 No 4
these X-rays are characteristic of the elements present in the sample and are determined by a spectrometer (wavelength dispersive) or a crystal detector (energy dispersive) depending on the nature of the sample. By making use of secondary electrons also generated in the process, a highly magnified image of the sample is produced. This enables excellent positional accuracy of the electron beam to be achieved. The following examples are included to provide a sample of the preparation methods used in dealing with III-V materials:
Electron Beam
BB
/~
.GaAssubstrata
Cleaved "~'~"~m-.,~ specimen ~ : 3 ~ i r ' / k
.---"S~ 'IR~--"~ _J'J.~" \ ~ I L " ~
Quanu tmwel \ Top
)Surface
F(gure 3. Schematic qfcleaved-ed~:e TEM method.
etching and analysis, producing the so called "Auger depth profile". In X-ray diffraction the incident Xrays are diffracted by the regular arrangement of atoms in a crystal. The intensity and angle at which diffraction occurs provides information about interatomic distances in the sample and enables us to define a range of compositional and structural information while X-ray topography can image the strain associated with defects in single crystals. Electron probe microanalysis utilises the X-rays generated in the top 13 microns of the sample during bombardment with a finely focussed beam of electrons. The energies of
Bevel sectioning In Fig. 1 a schematic diagram is shown of a typical advanced III-V laser structure. Characterisation of such a device would start in the early stages of production with qualification and quantification of the various layers and would include most of the techniques described in this article. Routine analytical requirements for quality control would typically require information on layer thickness and composition, lattice mismatch, homogeneity, crystalline quality and the integrity of heterointerfaces etc. AES can in addition be used to
produce in-depth compositional profiles. Of particular interest in this laser structure is the composition graded layer, which provides a graded refractive index of the confinement layers. The aim being to produce GaInAsP material that is linearly graded between the solid phase composition of InP and Ga0.181n0.82As0.40P0.60 whilst maintaining lattice matching conditions to the InP substrate. The method we have developed of quantifying the linearity of the grade is that outlined in Fig. 2, the so called "ion beam bevel section". This method uses a finely focussed ion beam electronically rastered in such a way as to sputter a very shallow linear bevel through the top layers to expose the region of interest. The bevel produces a magnified projection of the buried structure with layer magnifications of around 20,000 times. This method is used in combination with AES and enables the precise measurement of the composition of the buried layers. Compositional line scans across the region of interest are generated by scanning an electron beam across the bevel with the Auger spectrometer locked on to the element of interest. Fig. 2b shows the line scan for p h o s p h o r u s r e c o r d e d f r o m the
ANALYSIS of AI in AIGaAs CLEAVED EDGE TEM Dark Field g 200
AUGER PROFILE
image
AIGaAs InGaAs
E U:z E
10L
o-.e. E O
GaAs
10rim
.-
I
[
I
10
20 Depth in Layer nm
30
Figure 4a. Cleaved-edge TEM image of composition variation in AIGaAs layer. Figure 4b. Auger pro/ile showing extent of A I instability in A IGaAs layer
Page 43 I
Vol 6 No 4
graded region. Quantification of the grade is accomplished by taking selected points between its limits for a full analysis of all the elements. Fig. 2c is a band energy diagram of the planned structure with the Auger line scan excursion of Fig. 2b outlined. The a d v a n t a g e s of the bevelling approach over conventional Auger profiling are: • A high data density can be accumulated from the regions of specific interest, • Any number of elements can be profiled under optimised conditions, • Unsuspected contaminants can be found and identified, • The sample remains available for re-evaluation using the same bevel, • There is a faster sample turn-round than for conventional profiling.
Cleaved-edge TEM A fast accurate method of examining the epilayers in a III-V structure is the cleaved-edge TEM method. Fig. 3 shows how the method works. A small piece of wafer is polished from the back to a thicknes's of 100 p.m, and cleaved along < 1 1 0 > directions to form a small specimen with fresh, clean edges. The specimen is mounted on a copper grid as shown in the diagram, and this allows the electron beam of the TEM to penetrate the specimen near the edge where the thickness is less than 0.25 p.m. The image is recorded on film, and typical information which can be obtained from this technique includes: 1. The thickness of the epi-layers 2. The uniformity of the compositions 3. Whether the interfaces are fiat or uneven F'ig. 4a shows a TEM micrograph of an AIGaAs quantum well struclure. The thickness of the quantum well is easily measured to an accuracy of a few percent by this technique. However in this case, banding can be seen in the well structure. This is due to a regular variation of the composition through the well. TEM can give feedback within a few hours, and in this example uncovered a serious problem in the growth process. Fig. 4b is an Auger profile of the first few layers and shows the range of the AI instability in atomic %. The aluminium supply had developed an
Figure 5, Cros,s-seetion TEM o/quaternary &ser strueture.
instability, and after a short discussion with the growers the problem was identified, a change was made and the banding disappeared. The cleaved-edge TEM technique is a fast way of obtaining basic information about a III-V structure. No chemical staining is necessary and the sensitivity to composition and accuracy of measurement now make it the preferred technique over optical and SEM microscopies. When more detailed information is required, planview and cross-section TEM can be used.
Plan-view TEM In plan-view TEM a 3 mm disc of material is polished from the back to a thickness of 100 p.m, then chemically etched with a solution o f chlorine in methanol to perforation. The relatively large area near the hole can be examined for features such as dislocations and impurity phases. By tilting the specimen in the T E M and carefully selecting the diffraction condition used to form the image, dislocations can be analysed to determine their type and glide plane and from this information their source can often be determined.
Cross-section TEM Another method for examining the structures in III-V semiconductors is cross-section TEM. The unique feature of the procedure is the ability to look parallel to the growth interfaces Page 44
Vol 6 No 4
over a large distance with a resolution of 0.5 nm. In this case two pieces of material are glued face to face and polished perpendicular to the surface with a succession of SiC paper, alumina paste and diamond paste. This results in a 30 ~tm thick specimen highly polished on both sides with the epi interfaces perpendicular to the plane of the specimen. This is then ion milled to perforation, leaving thin area next to the hole suitable for TEM. Because of the instability of I I I - V c o m p o u n d s c o m p a r e d with silicon the ion milling is carried out at liquid nitrogen temperature to prevent dissociation of the material. Fig. 5 is a cross-section transmission electron micrograph from the quantum well structure whose compositional profile, analysed by Auger spectroscopy, was shown in Fig. 2. F r o m this cross section a b r o a d spectrum of assessment is possible. Structural quality such as the absence of dislocations, accurate measurement of layer thicknesses and the abruptness of the heterointerfaces in the multiple quantum well region can all be determined using this method. Typical problems that can be discovered by this method include: 1. Diffuse interfaces 2. Uneven layers - eg. if a layer has grown in a 3-D fashion 3. Dislocations generated within the structure 4. Compositional variations The examples presented in this article represent only a flavour of the wide ranging analysis required to fully characterise the complex layered structures on which modern III-V devices are based. It is essential that a range of complementary methods be available for such analysis. By the combined efforts of techniques such as multiple crystal x-ray diffraction and cross-section TEM for example, it is possible to measure the strain in these multi-layered structures and to show the position of misfit and threading dislocations which form to relieve the stress. Only with a full understanding of the elemental and physical composition can sufficiently accurate specifications for optimum device performance be determined. Contact: Peter Augustus. G M M T Ltd,, Caswell, Towcester, Northants, NN12 8EQ UK. Tel,(/a.w [44] ¢0)327 356362 356775 .